Systems and methods for forming foil contact rear emitter solar cell

ABSTRACT

A solar cell structure may provide a front surface that may include a front passivation layer and front anti-reflective layer. The solar cell structure may provide both contacts on a rear surface. In some cases, the rear surface may optionally provide passivation, doped, and/or transparent conductive oxide layers. The rear surface also provides a multilayer foil assembly (MFA). The MFA provides a first metal foil in electrical communication with doped regions of the rear surface of the substrate, such as base or emitter regions. The MFA may also provide a second metal foil that is spaced apart from the first metal foil by a dielectric layer. The first metal foil and/or the dielectric layer may include openings through the entirety of these layers, and these openings may be utilized to form laser fired contacts electrically coupled to the second metal foil, which is electrically isolated from the first metal foil. In some embodiments, it may be desirable for the second foil to provide openings as well, which can be utilized to form laser fired contacts for the first metal foil.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/210,230 filed on Aug. 26, 2015, which is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to all back contact solar cells. Moreparticularly, to systems and methods for fabricating back contact solarcells with multilayer metallization.

BACKGROUND OF INVENTION

A desirable solar cell geometry referred to as an interdigitated backcontact (IBC) cell comprises a semiconductor wafer and alternating lines(interdigitated stripes) respectively coinciding with regions withp-type and n-type doping. This cell geometry has the advantage ofeliminating shading losses altogether by putting both contacts on therear side of the wafer that is not illuminated. Further, contacts areeasier to interconnect with both contacts on the rear surface.

Another desirable solar cell architecture involves the use of siliconheterojunction or tunnel junction contacts. An example of sucharchitectures is the HIT (heterojunction with intrinsic thin layer) cellstructure. In a front emitter form of this structure, a silicon wafer iscontacted on both sides by a thin intrinsic hydrogenated amorphoussilicon (a-Si:H) layer, which serves as a surface passivating layer aswell as a charge carrier transport layer. On the front of the cell, asemiconductor layer doped to the opposite doping polarity of the basesubstrate is applied, forming a heterojunction emitter. On the rear ofthe cell, a semiconductor layer doped to the same doping polarity as thebase substrate is applied, forming a base contact. These layers can thenbe contacted with transparent or metallic conducting layers to extractcurrent from the solar cell. In the tunnel junction cell, the intrinsica-Si:H layer is replaced with a thin high bandgap material. In the caseof the heterojunction cell, charge carrier transport occurs via bothhopping conduction in traps and band conduction in the intrinsic a-Si:Hlayer, while in the case of the tunnel junction cell, charge carriertransport occurs via quantum mechanical tunneling. Despite thisdifference, the cell structures are somewhat similar and importantly canbe manufactured in low temperature processes because they do not requiredopant diffusion.

Heterojunction or tunnel junction solar cells cannot achieve outstandingefficiencies because they still require front side contacts. First, thepresence of a contact on the front side reduces efficiency due toblocking or shading of the incoming light by the necessary metal gridswhich extract the generated current. Additionally, the presence of afront electrical contact requires that the front of the cell besimultaneously optimized for electrical, light absorption, andpassivation properties, often producing a compromise that affects cellperformance.

Presently, silicon solar cells with the highest efficiency are thosebased on combining an interdigitated all back contact structure withsilicon heterojunction contacts. Silicon solar cells have been reportedwith efficiencies as high as 25.6%. While the processing of these highefficiency IBC solar cells were not discussed in any detail, themanufacturing costs are likely to be relatively high since the knownprocessing techniques that could be applied in each case appear to besomewhat complicated with various masking and vacuum processing stepsrequired.

Many solar cell structures, including the IBC structure, rely upon thinfingers of metal to collect current. In the case of the IBC cell, thesethin fingers are interdigitated on the back of the cell, and oftenresulting from a single metal deposition that has been patterned toreveal the finger structure. Since the fingers exist on the same layer,the area of each finger can only be about ½ of the full area of thecell, but actually less than ½ because there is need for an insulationregion (isolation gap) between the fingers. Combining this with the factthat current must travel along the narrow fingers, very highconductivity layers are required for the fingers. The solar industryresorts almost exclusively to silver, an expensive metal, to address theneed for high conductivity, thus low resistance. Additionally, largercell sizes exacerbate issues associated with resistance in the metalcontacts of the solar cell.

There have been attempts to provide improved metallization for solarcells. A multilayer metallization applies each contact metal (base,emitter) as a contiguous sheet of metal over the entire cell with viasin the bottom contact metal sheet allowing the top sheet to makelocalized base contacts, such vias made with a photolithographicprocess. However the photolithographic process is expensive and notsuited to solar cell manufacturing. Another method applies relativelythick metals in cost effective ways, but does not address approachesneeded for make useful back contact cells.

Therefore, there is a need to provide multilevel metallization systemsand methods for back contact solar cells that can be low cost. In U.S.patent application Ser. No. 15/068,900 filed on Mar. 14, 2016, laserprocessed back contacts were discussed. Further improvements to systemsand methods for forming rear emitters for solar cells are discussedherein.

SUMMARY OF THE INVENTION

In one embodiment, roll-to-roll processing comprises multiple laminationor merging steps to form the layers of a multilayer foil assembly (MFA).These lamination steps may merge a dielectric insulating layer to one ormore metal foil layers. In some cases, the lamination steps may includeother layers, such as bonding layers or the like. The roll-to-rollprocessing may also include steps performed at various stages to formdesired openings in one or more layers of the multilayer foil assembly,such as by laser drilling of via arrays, that are aligned with dopedregions of a substrate. It may also be desirable to perform openingformation steps to provide fiducial holes that may be utilized to aidaligning layer(s). In some embodiments, the roll-to-roll processing mayalso involve high-speed dopant printing (e.g. inkjet, aerosol jet, lasertransfer printing, etc.) to deposit one or more dopant materials atdesired locations. Once the multilayer foil assembly foil assembly iscompleted, the multilayer foil assembly can be bonded to a silicon waferfor a laser firing/doping process to form laser fired contacts for baseregions, emitter regions, or both. With the patterning provides for theone or more layers of the MFA, each the metal foil layers can berespectively coupled to base regions or emitter regions while beingelectrically isolated from each other.

In another embodiment, a solar cell structure may provide a frontsurface that may include a front passivation layer and frontanti-reflective layer. The solar cell structure may provide bothcontacts on a rear surface. In some cases, the rear surface mayoptionally provide passivation, doped, and/or transparent conductiveoxide layers. The rear surface also provides a multilayer foil assembly.The multilayer foil assembly provides a first metal foil in electricalcommunication with regions of the rear surface. The multilayer foilassembly may also provide a second metal foil that is spaced apart fromthe first metal foil by a dielectric layer. The first metal foil and/orthe dielectric layer may include openings through the entirety of theselayers, and these openings may be utilized to form electrical contactssuch as laser-fired contacts. In some embodiments, it may be desirablefor the second foil to provide openings as well, which can be utilizedto form laser-fired contacts.

The foregoing has outlined rather broadly various features of thepresent disclosure in order that the detailed description that followsmay be better understood. Additional features and advantages of thedisclosure will be described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIG. 1A shows an exploded view of an illustrative embodiment of amultilayer metallization scheme;

FIGS. 1B-1C show illustrative embodiments of different doping patterns;

FIG. 2 shows a cross-section view of an illustrative embodiment of apartially assembled solar cell;

FIG. 3 is an illustrative embodiment of a process flow forming a solarcell;

FIG. 4 is a cross-section view of an illustrative embodiment of apartially assembled solar cell;

FIG. 5 is a cross-section view of an illustrative embodiment of apartially assembled solar cell;

FIG. 6 is a cross-section view of an illustrative embodiment of apartially assembled solar cell;

FIG. 7 is a cross-section view of an illustrative embodiment of a solarcell;

FIG. 8 is an illustrative embodiment of a process flow forming a solarcell with multilayer metallization;

FIG. 9 is a cross-section view of an illustrative embodiment of a solarcell with a foil assembly prior to assembly;

FIG. 10 is a cross-section view of an illustrative embodiment of a foilassembly with an overhang;

FIG. 11 is an illustrative embodiment of a process flow forming a solarcell with multilayer metallization;

FIG. 12 is a cross-section view of an illustrative embodiment of a solarcell with a foil assembly prior to assembly;

FIG. 13 is a cross-section view of an illustrative embodiment of a solarcell with a heterojunction;

FIG. 14 is a cross-section view of an illustrative embodiment of a solarcell with p⁺ and n⁺ laser fired contacts;

FIG. 15 is an illustrative embodiment of a process flow forming a solarcell with high volume multilayer foil assembly;

FIG. 16 is a cross-section view of an illustrative embodiment of a solarcell prior to high volume multilayer foil assembly;

FIG. 17 is a cross-section view of an illustrative embodiment of a solarcell with a roll-to-roll prepared multilayer foil assembly;

FIG. 18 is an illustrative embodiment of a roll-to-roll process forforming a multilayer foil assembly;

FIG. 19 is a cross-section view of an illustrative embodiment of a solarcell formed utilizing a roll-to-roll prepared multilayer foil assembly;

FIG. 20 is another illustrative embodiment of a roll-to-roll process forforming a multilayer foil assembly that include dopant deposition;

FIG. 21 is a cross-section view of an illustrative embodiment of a solarcell formed utilizing a roll-to-roll prepared multilayer foil assemblythat includes laser fired p⁺ and n⁺ contacts;

FIG. 22 is a cross-section view of another illustrative embodiment of asolar cell formed utilizing a roll-to-roll prepared multilayer foilassembly that includes laser fired p⁺ and n⁺ contacts;

FIG. 23 is an illustrative embodiment of a roll-to-roll process forforming a multilayer foil assembly including a second array of viasand/or dopant deposition;

FIG. 24 is a cross-section view of an illustrative embodiment of a solarcell with nickel silicide formed utilizing a roll-to-roll preparedmultilayer foil assembly;

FIG. 25 is an illustrative embodiment of a roll-to-roll process forforming a multilayer foil assembly with a second array of vias and/ordeposition of a metal contacting layer and dopant;

FIG. 26 is a cross-section view of an illustrative embodiment of a solarcell with a multilayer foil assembly prior to contact formation;

FIG. 27 is a cross-section view of an illustrative embodiment of a solarcell with a multilayer foil assembly during contact formation;

FIG. 28 is a cross-section view of another illustrative embodiment of asolar cell with a multilayer foil assembly prior to contact formation;

FIG. 29 is a cross-section view of another illustrative embodiment of asolar cell with a multilayer foil assembly during to contact formation;

FIG. 30 is a cross-section view of another illustrative embodiment of asolar cell with a multilayer foil assembly prior to laser-fired contactformation;

FIG. 31 is a cross-section view of another illustrative embodiment of asolar cell with a multilayer foil assembly after laser-fired contactformation;

FIG. 32 is a cross-section view of an illustrative embodiment of apre-assembly solar cell with a multilayer foil assembly providing foiltexturing;

FIG. 33 is a cross-section view of an illustrative embodiment of a solarcell with a multilayer foil assembly with foil texturing after assembly;

FIG. 34 is a view of an illustrative embodiment of solar cells connectedin series; and

FIG. 35 is an illustrative embodiment of a solar cell structure with amultilayer foil assembly.

DETAILED DESCRIPTION

Refer now to the drawings wherein depicted elements are not necessarilyshown to scale and wherein like or similar elements are designated bythe same reference numeral through the several views.

Referring to the drawings in general, it will be understood that theillustrations are for the purpose of describing particularimplementations of the disclosure and are not intended to be limitingthereto. While most of the terms used herein will be recognizable tothose of ordinary skill in the art, it should be understood that whennot explicitly defined, terms should be interpreted as adopting ameaning presently accepted by those of ordinary skill in the art.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory only,and are not restrictive of the invention, as claimed. In thisapplication, the use of the singular includes the plural, the word “a”or “an” means “at least one”, and the use of “or” means “and/or”, unlessspecifically stated otherwise. Furthermore, the use of the term“including”, as well as other forms, such as “includes” and “included”,is not limiting. Also, terms such as “element” or “component” encompassboth elements or components comprising one unit and elements orcomponents that comprise more than one unit unless specifically statedotherwise. The following general description sections are applicable tothe various embodiments discussed further herein.

General Description Multilayer Metallization:

A nonlimiting example of an exemplary solar cell with multilayermetallization is described herein and is generally applicable to thevarious embodiments discussed further herein. FIG. 1 shows an explodedview of a multilayer metallization scheme applicable to the variousembodiments discussed further herein. A substrate (100) has on itssurface a first metal layer or metal foil layer (110) representing afirst electrical contact, a dielectric layer (115) and a second metallayer or metal foil layer (120) representing a second electricalcontact. The dielectric layer is sandwiched between the first and secondmetal foils. Electrical current generated from the cell can be extractedfrom first and second electrical contacts. The dielectric layer (115)serves to insulate the first and second conductive layers, which arerespectively coupled to different doping regions. While the explodedview shows the layers separated, it shall be understood that the layersmay be in contact in the order shown.

A semiconductor substrate (100) useful in a solar cell can have regionsof different doping or polarity, typically referred to n-type or p-typeregions, and a general doping type of the base substrate, referred to asthe base doping. For ease, regions of doping that are the same as thebase doping type can be referred to as the base, and regions that are ofthe opposite doping type can be referred to as the emitter. These dopingregions may be arranged in a pattern suitable for forming interdigitatedback contacts (IBCs). A built-in potential exists between the emitterand base regions making it desirable to form IBCs coupled to the emitterand base regions, such as from the first and second metal layers 110,120. The operation of the solar cell is achieved by illuminating thesemiconductor and collecting the current driven by the built-inpotential by connecting one of the metal contacts to one doping region,and the other metal contact to the other doping region. As a nonlimitingexample, region (155) of a first doping type (e.g. n-type or p-type) isconnected to a region (150) on the first metal contact. Region (145) ofsecond doping type, which is the opposite of region (155), is connectedto region (140) of the second metal contact. The doping regions (145)and (155) may be formed on the substrate prior, during, or after theapplication of the metal contact layers, or as a result of the metalcontact application process. The substrate (100) may also contain otherfunctional layers such as conductors, semiconductor, and passivationlayers.

In order for the second metal layer (120) to properly operate and makecontact with the substrate (100) without making contact to the firstmetal layer (110), the first metal layer may have openings (130) alignedwith the location of the second metal contact (140,145). The dielectriclayer may also have similarly aligned openings 135. The openings may beof any suitable shape, including circular, square, oval, rectangular, orthe like. The latter can be useful in the case of laser processing usingan elongated laser beam shape.

Many of the figures discussed herein are shown in cross sectional viewsfor illustrative purposes. For example, the cross sectional views may befrom positions (122) or (124). It should be understood that the crosssection is for illustrative purposes and in all cases the individuallayers may be contiguous layers as shown in FIG. 1.

The first metal layer (100) and second metal layer (120) may be metalfoils that comprise thin continuous sheet sections of flexible metal,which, at some point in the process, may have been freestanding thinlayers of metal. As utilized herein a multilayer foil assembly or metalfoil assembly (MFA) may be utilized interchangeably. A multilayer foilassembly or metal foil assembly refer to one or more layers of metalfoil(s), dielectric layer(s), or a combination thereof, which may beutilized in the various processing methods discussed herein for formingsolar cells. As a nonlimiting example, one or more of the metal foilsmay be a component of a metal foil assembly, which may include one ormore material layers, such as the metal foil and a dielectric layer. Insome embodiments, the materials layers of the MFA, such as metal foil(s)or dielectric/insulator layer(s), are freestanding layer(s) rather thanlayers formed by depositing materials on a substrate. The MFA may beformed as an assembly prior to bonding to a substrate, or the metal foilassembly may be formed and result upon the substrate by the successiveplacement and bonding of several layers. These layers formed upon thesubstrate may themselves be metal foil assemblie(s), and the metal foilassemblie(s) may be bonded to other metal foil assemblie(s) or otherlayers that are joined to create a new metal foil assembly.

General Description of Substrate:

A general description of a substrate for the various embodimentsdiscussed further herein is provided below. A more detailed view of anonlimiting example of a substrate (100) composed of a basesemiconductor (160) is shown in FIG. 2. The substrate may have a frontsurface (164) for receiving light and a rear surface (166) containingactive layers. The front surface (164) may be textured (190) to improvelight absorption, and may have various passivation (170) andantireflection (180) layers, as is described in more detail below. Onthe back surface or back side, the substrate (100) may have variousdoping or junction inducing layers (235) represented by exemplaryindividual layers (200,210). The back side may also have conductivelayer(s) (240), as described in more detail below. It is understood thatthe various layers can be applied by any suitable fabrication processand at any suitable point in the fabrication process.

The suitable substrate (100) may be a semiconductor wafer of anyconvenient size or shape. Nonlimiting examples of suitablesemiconductors include group IV semiconductors, such as silicon orgermanium; group III-V semiconductors, such as gallium arsenide orindium phosphide; and group II-VI semiconductors, such as cadmiumtelluride. In some embodiments, the substrate (100) thickness is equalto or below about 1 mm. The surface of the semiconductor wafer may bepolished. In some embodiments for solar cell applications, the startingwafer may have a surface that is textured (190) to promote lightabsorption. The surface texture (190) may be applied by mechanicalmeans, laser processes, chemical etching processes, or the like. In someembodiments for silicon wafers, the surface texture (190) may containexposure of predominantly <111> and <110> facets, such as is obtained bytreatment with solutions containing KOH or NaOH in conjunction withsurfactants such as alcohols. In some embodiments, the front surface(164) may be chemically smoothed by treatment with various etchants. Astarting wafer that has a rough texture resulting from the wafer sawingprocess can be chemically smoothed with hydroxide containing etchants,such as NaOH, KOH, TMAH (tetramethylammonium hydroxide), combinationsthereof, or the like. The concentrations of these etchants can be equalto or greater than 10%, and etching may be performed at temperaturesequal to or greater than 50° C. The resulting surface (164) may besubstantially smoothed relative to the starting rough surface, but maystill contain pits, depressions, or surface undulations.

As a nonlimiting example, the surface (164) of the wafer may be texturedwith very fine features to produce a gradient refractive index, alsoreferred to as a nanoscale texture or black silicon. In someembodiments, the anti-reflective etching process may be a single stageprocess that includes a catalytic metal and etching chemistries. In someembodiments, the anti-reflective etching process may be a multi-stageprocess that includes: a catalytic metal deposition stage to deposit ametal on the substrate, and an etching stage that texturizes surface(s)of the substrate to reduce reflectivity. In some embodiments, thecatalytic metal deposition stage may occur utilizing a thin layer fluidprocess that includes steps similar to the etching stage as discussedfurther herein. In some embodiments, the catalytic metal may exist in adeposition fluid as a precursor that is reduced or plated on thesubstrate surface. As a nonlimiting example, catalytic metal solutionscontain a catalytic metal and a fluorine containing compound, such ashydrofluoric acid, that is dispensed and/or dispersed into a thin fluidlayer on the substrate to deposit the catalytic metal on the substrate.In some embodiments, the deposition fluid is dispersed or spread outinto the thin layer with a thickness of 5 mm or less. In otherembodiments, the deposition fluid is dispersed or spread out into thethin layer with a thickness of 1.5 mm or less. A thickness of the thinlayer of deposition fluid may be controlled by controlling a separationdistance between the first surface of the substrate and an opposingsurface of a dispersion mechanism opposite the first surface. Thedeposition fluid may remain in contact with the substrate for about 5seconds to 5 minutes for the catalytic metal deposition stage. After thecatalytic metal deposition stage, the metal catalyst has been depositedon portions of the substrate (100), and the anti-reflective etchingprocess may proceed to the etching stage to texturize (190) the surfaceof the substrate where the metal catalyst was deposited on thesubstrate.

In some embodiments, the substrate (100) may have different surfacetextures on the front and rear surfaces. The front surface may betextured (100) to promote light absorption, while the rear surface maybe textured or smoothed to promote compatibility with the contacting andlaser firing processes. The front surface may have textures (100) asdiscussed above while the back surface may be a nominally smooth surfaceobtained by mechanical polishing, chemical mechanical polishing (CMP),or chemical etching. Differential textures on front and back may beachieved by any suitable means. The different surfaces may be subjectedto different treatments by protecting one of the surfaces with aprotective coating while immersing the substrate in a treatment bath orprocessing in a chamber. The substrate may be subjected to a single sideprocess by maintaining the substrate partially immersed in a fluid,where one face is immersed and the other face is not. Alternatively, thesubstrate may be processed in a chamber where only one side of thesubstrate is treated.

The starting substrate (100) may be highly pure, and thus nearlyintrinsic in doping character, or may have a particular bulk dopingleading it to be n-type or p-type. This presence of doping modifies thebulk resistivity of the substrate. In some embodiments, substrates mayhave a bulk resistivity equal to or between about 0.1 to 50 ohm-cm. Insome embodiments, substrates may have a bulk resistivity equal to orbetween about 1 to 25 ohm-cm. In some embodiments, the substrate may ben-type doped silicon grown by the Czochralski method. The examplesdiscussed above and herein are provided for illustrative purposes only,and it will be recognized that substrates are in no way limited to theparticular examples discussed.

General Description of Front Passivation:

Front passivation layers (170) may be applied by any suitable means. Asa nonlimiting example, the front passivation may include a process suchas atomic layer deposition (ALD). The material deposited by ALD mayinclude aluminum oxide (Al₂O₃) or silicon dioxide (SiO₂). As anonlimiting example, the passivation process may include ALD depositionof aluminum oxide using trimethylaluminum (TMA) as a precursor. Thesubstrate (100) may be annealed after deposition of the ALD depositedlayer to improve or alter the passivation quality. As anothernonlimiting example, the front passivation (170) may be achieved byexposing the substrate to oxygen at elevated temperature to produce athermal oxide.

In some embodiments, the front passivation (170) may be a semiconductorlayer. A nonlimiting example of a semiconductor passivation layer ishydrogenated amorphous silicon (a-Si:H). The a-Si:H may be deposited byany suitable means, including plasma enhanced chemical vapor deposition(PECVD) or hot wire chemical vapor deposition (HWCVD). In someembodiments, the deposition may take place at temperatures ranging fromequal to or between approximately 150° C. to 450° C., or from equal toor between approximately 200° C. to 400° C. The a-Si:H passivation maybe undoped, indicating that no intentional doping compounds areincluded. Alternatively, the a-Si:H passivation layer may be lightlydoped by a doping compound. In some embodiments, the a-Si:H layer isrelatively thin (e.g. equal to or between about 2 to 20 nm) to minimizelight absorption at the front surface. The front passivation may includeseveral layers of a-Si:H or other semiconductor materials with variousdoping levels. In some embodiments, a passivation (170) structure maycontain a first layer of intrinsic a-Si:H in contact with the siliconsubstrate and a second layer of doped semiconductor such as dopeda-Si:H. These and other structures may produce a front surface field toprevent charge carrier recombination at the front surface.

General Description of Doping Regions:

A solar cell may have doping regions (e.g. 145, 155 in FIG. 1A)exhibiting a particular doping characteristic. The doping characteristicmay be defined by parameters including, but not limited to, the dopingpolarity (n-type or p-type), the level or concentration of dopant atoms,and the Fermi level. The doping regions may be created by methodsincluding incorporation of dopant atoms in the substrate, application oflayers on the substrate that modify charge carrier concentrations ormovement in the substrate, and may be created during any point in thecell fabrication process. In some embodiments, a doping region may becreated before (e.g. FIG. 9), during, or after (e.g. FIG. 13) theapplication of the multilayer foil, and different doping regions may becreated at different points in the process Like interdigitated backcontact (IBC) solar cells, it is desirable to have a contact inelectrical connection with a first doping region, and another contact inelectrical contact with a second doping region that is electricallyisolated from the first contact.

A doping region may occupy part or all of the surface of the substrate(e.g. 235 in FIG. 2). A doping region may be created by processing partor all of an existing doping region, thus modifying its dopingcharacteristics. In some embodiments, a substrate may have a firstdoping region (e.g. 155 in FIG. 1A) covering a portion of the substrate,and a second doping region (e.g. 145) of opposite polarity may becreated within the first doping region by locally modifying the regionwith a laser or other process. In one embodiment, a substrate may have afirst doping region that contains a given concentration of dopantproducing a first polarity. Second doping regions comprising localizedareas may be produced by a laser doping process that locallyincorporates sufficient dopant capable of reversing the polarity suchthat second doping regions are created despite the presence of theoriginal dopant. Doping in the first and second regions may beunderstood to include doping by local incorporation of dopant or dopingin heterojunction or adjacent layers. In further discussion below, itwill be apparent that the various processing options may produce avariety of doping patterns on the substrate. FIGS. 1B-1C showillustrative embodiments of different doping patterns that may beproduced.

FIG. 1B is a top view of a substrate (100) that contains a first dopingregion (157) of a first doping characteristic. The patterning of FIG. 1Bmay result when the wafer selected for processing is already doped (e.g.polarity corresponding to first doping region (157)). The substratefurther contains a second doping region (147) of a second dopingcharacteristic patterned or aligned with metal layer openings (130) anddielectric layer openings. Doping regions (147) can be referred to aslocalized contacts. In some embodiments, each doping region (147) maycomprise one or more laser fires. The first doping region (157) can becontinuous or contiguous, such that each second doping region (147) isencircled or surrounded by the first doping region in the plane of thesubstrate. It shall be understood that encircled or surrounded byindicates the perimeter of the second doping regions (147) aresubstantially surrounded by the first doping region (157). In someembodiments, encircled or surround by may mean that a radius from thesecond doping region exists such that greater than 80%, or preferablygreater than 90% of the resulting circumference (e.g. exemplified by136), is occupied by the first doping region. In contrast, conventionalinterdigitated back contact designs provide doping regions where thefingers interface into gaps of opposing finger, but none of the dopingregions surround the other doping region. The projection of the openingsin the metal foil assembly, for example the first metal foil opening(130), show that the openings should be sized properly to avoidelectrical contact between the metal foil layers (110, 120) and/ordoping regions (147,157). As a nonlimiting, the first doping region canextend to within the position of the openings, although that is notrequired. It is contemplated also that a region of intermediate dopingor light doping (148) can exist between the first and second dopingregions. In some embodiments, a first doping region (157) is inelectrical contact with a first metal foil and a second doping region(147) is in electrical contact with a second metal foil. In thisembodiment, the first metal layer can be continuous or contiguous in asimilar manner as the first doping region (157), such that each seconddoping region (147) is encircled by the first metal layer, but not inelectrical contact with the first metal layer. In some embodiments,encircled may mean that a radius from the second doping region existssuch that greater than 80%, preferably greater than 90% of the resultingcircumference (exemplified by 136) is occupied by the first metal layer.

FIG. 1C is a top view of a substrate (100) that contains localized firstdoping regions (158) of a first doping characteristic in addition tosecond doping regions (147) as described above. The patterning of FIG.1C may result when the wafer selected for processing is undoped orlightly doped. Doping regions (158) can be referred to as localizedcontacts. In some embodiments, each doping region (158) may comprise oneor more laser fires. In some embodiments, doping regions (158) may becreated by laser firing through a first metal layer, using the metal ofthat layer as a dopant, or using a material coated on that layer as adopant. In some embodiments, a first doping region (158) is inelectrical contact with a first metal foil. Second doping regions (147)are similar to those described in the prior embodiment. Second dopingregions (147) are arranged to be in electrical contact with a secondmetal foil in the same manner as discussed previously above. However,because the first doping regions (158) do not encircle or surround thesecond doping regions (147) in the manner described above, a largeundoped region or lightly doped (159) is present.

Regarding the novelty of the patterning in FIGS. 1B-1C, in aconventional interdigitated finger design of an all back contact cell,one series of metal fingers would need to contact to the first dopingregions, while another set of metal fingers patterned on the same planeand isolated from first set by a gap or insulating material would needto contact the second doping regions. It can be seen from study of theexample multi-layer arrangement and patterns in FIGS. 1A-1C that ratherthan isolating sets of fingers via a gap or insulating material in thesame plane as the fingers, the present design isolates using the metalfoil opening design discussed.

In some embodiments, the first doping region and the second doping mayhave additional electrical isolation features to avoid significant shuntcurrent. Electrical isolation can be achieved by physical separation ofthe doping regions with portions of the substrate that have a low dopinglevel. Electrical isolation can also be achieved by relying upon thedoping characteristics of the first and second doping regions to producea rectifying junction that does not allow substantial photogeneratedcurrent to pass. Isolation can be achieved by a process steps including,but not limited to, laser processing, laser etching, wet etching, or dryetching.

General Description of Rear Emitter:

The rear emitter may be composed of one or more layers (235) that modifythe charge carrier concentrations in a portion of the rear of the cell.For example, a rear emitter may be composed of a heterojunction (aninterface that occurs between two layers or regions of dissimilarsemiconductors) or tunnel junction (a barrier, such as a thin insulatinglayer or electric potential, between two electrically conductingmaterials). These structures may be applied by any suitable means. Aheterojunction layer may comprise any suitable semiconductor orsemiconductor layers with an appropriate band structure, mechanicaland/or adhesion properties. In some embodiments, the heterojunctionlayer is formed by the application of a passivation layer (200) and adoped layer (210). As a nonlimiting example, the passivation layer maybe intrinsic or very lightly doped a-Si:H. The doped layer may be dopeda-Si:H with any dopant or dopant concentration that is suitable formodifying charge carrier concentrations in the solar cell. The intrinsicor lightly doped a-Si:H layer can be any suitable thickness thatpromotes good charge carrier transport while maintaining integrity. Insome embodiments, the thickness can be in the range equal to or betweenapproximately 2 to 20 nm, or equal to or between approximately 4 to 10nm.

A rear doped semiconductor layer (210) may be applied by any suitablemeans. A rear doped semiconductor layer (210) may comprise any suitablesemiconductor with the appropriate band structure, mechanical, and/oradhesion properties. As a nonlimiting example, the rear dopedsemiconductor layer (210) may be doped silicon. The silicon may beamorphous, microcrystalline, or polycrystalline depending upon growthconditions. The silicon layer may be deposited by any suitable means,including plasma enhanced chemical vapor deposition (PECVD) or hot wirechemical vapor deposition (HWCVD). In some embodiments, the depositiontakes place at temperatures ranging equal to or between approximately150° C. to 450° C., or equal to or between approximately 200° C. to 400°C. Doping may be accomplished by including a chemical dopant in thedeposition feed stream. Nonlimiting examples of dopant chemicals includeborane, diborane, phosphine, and/or arsine. The dopant may be present inthe feed stream at concentrations from equal to or between approximately0.5% to 20% on a molar basis relative to the silicon precursor. In someembodiments, dopant concentrations are from equal to or betweenapproximately 2% to 10%.

The passivation layer (200) may also be any suitable insulator or largeband-gap material with the appropriate band structure, mechanical,and/or adhesion properties. With certain types of insulators or highbandgap semiconductors utilized as the passivation layer (210), theapproach shown in FIG. 2 may be termed a tunnel junction. Nonlimitingexamples of tunnel junction materials are SiO₂, SiN_(x), or Al₂O₃. Atunnel junction may be applied by ALD, PECVD, thermal oxidation (whereapplicable), or chemical oxidation (where applicable).

In some embodiments, the rear emitter may be formed by modifying theproperties of the silicon layer (160) at its rear surface or within. Forexample, a rear emitter may be formed by doping the rear surface regionof the silicon layer (160) with the appropriate atom. Nonlimitingexamples of suitable dopants are aluminum, gallium, boron, phosphorous,antimony, and arsenic. The doping may be accomplished with a diffusionprocess, in which the substrate (160) is heated to a predeterminedtemperature in the presence of the dopant or a precursor to the dopant.In some embodiments, the diffusion may occur at temperatures rangingfrom 600° C. to 1100° C., and times ranging from 10 minutes to 2 hours.In some embodiments, doping may also be accomplished with ionimplantation followed by high temperature annealing. As a nonlimitingexample, an emitter may be formed by doping a portion of the substrateto form the desired doped region, as opposed to depositing exemplarylayers (200) and (210).

It is understood that abovementioned examples are merely illustrative,and the rear emitter forming layers (235) can be accomplished with anysuitable selection of materials or processes that accomplish emitterbehavior.

General Description of Rear Passivation:

In some embodiments, the rear surface (166) of solar cell is passivatedto provide a low surface recombination velocity (SRV) surface, and adoping process can be performed later in the process to produce anemitter function. In that case, layer combination (235) may be composedof one or more layers and can serve as a passivation structure and canhave similar properties as described for the front passivation (170).The rear passivation may include layers which also modify the chargecarrier concentrations in that portion of the cell. Such layers maycontain a fixed charge that interacts with charge carriers in thesubstrates, possibly producing inversion or accumulation layers. Therear passivation layers may have similar properties and compositions tothose described above for front side passivation.

General Description of Rear Conductive Layers:

A rear conductive structure (240) may be applied to the back of thecell. In some embodiments, the conductive structure may comprise anysuitable conductive material, including metals, highly dopedsemiconductors, and transparent conducting oxides (TCO). Further, someembodiments may utilize combinations of these materials may be used. Asa non-limiting example, a layer of a TCO followed by a layer of metalmay be used. The TCO/metal combination may impart improved backreflection to the solar cell; thus, improving light capture andperformance. In some embodiments, the rear conductive structure (240)may include additional conductive layers that are applied in subsequentprocessing.

Nonlimiting examples of TCOs include indium tin oxide, doped zinc oxide,and/or doped tin oxide. Nonlimiting examples of dopants includealuminum, gallium, and/or fluorine. Nonlimiting examples of metalconductive layers include aluminum, copper, nickel, silver, gold, and/orantimony. TCOs and metals may be deposited by any suitable depositiontechnique. Nonlimiting examples of deposition methods for TCOs mayinclude sputtering, plasma enhanced chemical vapor deposition, and/oratomic layer deposition and for metals may include thermal evaporation,electron beam evaporation and sputtering.

The thickness of a TCO, that may comprise one element of rear conductivestructure (240), may be in the range of equal to or between 10 to 1000nm, or equal to or between 30 to 200 nm. The thickness of a metal layer,that may comprise one element of rear conductive structure (240), may bein the range of equal to or between 10 to 4000 nm, or equal to orbetween 20 to 2000 nm. It one embodiment the rear conductive structure(240) comprises a TCO layer followed by a thin layer of metal, in whichthe thin metal layer aids in providing good electrical contact tosubsequently applied metal foil assemblies discussed herein.

Etched Metal:

In some embodiments discussed further herein, a solar cell withmultilayer metallization is fabricated by patterning one of the metallayers with an insulating dielectric that is used in the fabrication ofthe MFA, such as the dielectric that insulates the first and secondmetal layers from each other.

Referring to the process flow in FIG. 3, a silicon wafer is selected(S250) and is prepared by etching, cleaning, and texturing steps (S260)as described above. In steps (S270) and (S280) emitter or passivationlayers are applied as discussed above.

Referring to process flow in FIG. 3 and device structure in FIG. 4, afirst metal layer (230) is applied to the rear of the solar cell (S290).The metal may be applied by any suitable deposition technique, includingvacuum deposition or atmospheric deposition. Nonlimiting examples ofvacuum deposition steps include sputtering, thermal evaporation, andchemical vapor deposition. Nonlimiting examples of atmosphericdeposition steps include atmospheric chemical vapor deposition, aerosoldeposition techniques, arc spraying and solution deposition or liquidcoating techniques.

The deposited metal may be any metal with suitable conductivity, or amixture of metals. In some cases, it may be useful to deposit metal in asequence of layers for improved conductivity and/or adhesion.

The metal may be applied as metal foil or metal foil assembly, andconductively bonded to the substrate by any of the techniques discussedbelow. The foil may be of any thickness that provides suitableelectrical conductivity and mechanical properties. In some embodiments,the foil thickness can be in the range equal to or between 2 to 100microns, or in the range equal to or between 10 to 40 microns.

Any metal with suitable electrical and/or mechanical properties may beused as the foil. Nonlimiting examples of these metals include aluminum,copper, silver, gold, or antimony. Further, metal alloys may be used toimpart the desired properties. The metal foil may comprise multiplelayers of metal and/or other materials (e.g. bonding or dielectriclayers).

In some cases, the process of conductive bonding of the foil to the rearof cell may provide sufficient electrical connection, such as for asubstrate that contains an emitter on the rear side. In someembodiments, laser firing through the 1^(st) foil and passivation layermay optionally be used to create point doping contacts when desired(S300), such as in cases where the substrate include surface passivationon the rear of the cell. In such cases, the doping material may comefrom the foil or a doping substance coated on the foil. Even if asuitable emitter or sufficient electrical connection exists on the rearside of the wafer from prior processing, laser point firing mayoptionally be used as well to improve contact to the emitter. Furtherdetails of such laser firing is discussed below in the laser processingsection.

A dielectric layer (350) may be applied over the metal (S310). Thedielectric insulator can be any suitable material which has lowelectrical conductivity and can be patterned. In some embodiments, thepatterning of the dielectric layer may occur during deposition.Nonlimiting examples of deposition processes may include screenprinting, inkjet printing, and gravure coating. In some embodiments,patterning may be done after deposition. For example, post depositionpatterning may include light based exposure and developing, alsoreferred to photoresist processing, and laser ablation techniques. Dyesand other compounds may be added to the dielectric layer to promoteimproved removal in the above processes.

In some embodiments, the dielectric layer may include epoxies, acrylics,polyesters, polyvinyl butyrals, and polyvinyl alcohols. It may be coatedwith any suitable solvent, including water, cyclohexanone, propyleneglycol monomethyl ether, acetone, methyl isobutyl ketone, and/or methylethyl ketone.

The dielectric layer may be of any thickness suitable to ensure a highelectrical resistance. In some embodiments, the dielectric thickness mayrange from equal to or between 0.1 microns to 100 microns, or equal toor between 5 microns to 50 microns.

Referring to process flow in FIG. 3 and device structure in FIG. 5, thepatterning of metal layer (230) can be accomplished by etching, wherethe patterned dielectric layer (350) acts as a mask (step S320). Inembodiments other than FIG. 5, patterning of the metal layer can beperformed by perforating, drilling, or any other suitable patterningmethod. This etching process only removes metal layer (230) in locations(360) where the dielectric layer (350) is not present. This “selfalignment” has the benefit that, regardless of distortions and errors inthe pattern of the dielectric (350), the metal (230) will be etched inthe same pattern. The metal (230) may be in contact with first dopingregions, whereas the patterning of the openings in dielectric (350) andmetal (230) correspond to second doping regions, similar to thearrangement in FIG. 1A.

The etching process may be designed to remove additional layers such aspart or all of the previously applied rear conductive structure (240).Additionally, it is contemplated that the etch process may remove oralter some or all of the emitter forming layers (235) in the region ofthe dielectric openings. In some embodiments, the etching process mayremove some or all of the rear doped semiconductor layer (210) to allowthe substrate under the opening (360) to remain passivated withpassivation layer (200) with similar properties to the silicon base(i.e., not function as an emitter area).

During the etching process (S320) additional regions (370) of the metallayer (230) may be protected with an additional masking layer (notshown) or may not be exposed to the etch system so as to yield exposedmetal not covered by dielectric for subsequent contacting purposes.

The etching of the metal may be accomplished via solution means usingany standard etchant compositions for the desired metal. The etchantsolution may be water based and may contain any combination of acids,oxidizers, and bases required to perform etching at a suitable rate.

Referring now to process flow in FIG. 3 and device structure in FIG. 6,the etching process (S320) may be optimized to produce an overhang orundercut (365) of the conductive and other functional layers so thatsubsequent layers are prevented with more efficacy from makingundesirable contact to the metal layer (230). This overhang or undercut(365) may be described as etching that results in the dielectric layer(350) extending over one or more layers (e.g. layers 230, 240) below it.

Referring now to process flow in FIG. 3 and device structure in FIG. 7,a supplementary insulating layer (355) as described below can optionallybe applied (S330) to aid in preventing undesirable contact to the firstmetal layer (230). In the next step, laser base doping (S340) can beperformed in order to produce base doping contact points or laser firedcontacts (364) which can be used to allow a second metal layer (362) tomake contact to the base (160). Laser doping can be performed asdescribed further below in the laser processing section.

The second metal (362) can be applied by vacuum or atmospheric processesas described above. In some embodiments, the second metal may be appliedas a metal foil with contacting methods as described below.

Patterned Foils:

Embodiments involving the use of two overlapping metal foils, forexample, but not limited to, two aluminum foils, separated by aninsulating layer to form a back-contact solar cell are discussed herein.

In some embodiments, the layer(s) of a MFA may be applied individuallyto the rear surface of the substrate. In some embodiments, the firstmetal foil may be perforated to provide patterned openings or an arrayof holes at desired regions, such as openings corresponding to firstdoping regions or second doping regions. As a nonlimiting example, thefirst metal foil may be perforated by drilling an array of small holesin the foil before placement on rear surface of a passivated wafer.These holes would provide vias for laser firing contacts using thesecond metal foil as a dopant source or a dopant layer coated on thesecond metal foil. A dielectric isolation layer between the two foilscould be provided by coating one of the foils with an insulating layeror by anodizing one surface of one of the foils. While an Al foil canact as a p+ dopant source in some embodiments, an n+ dopant source couldbe coated on one surface of one of the foils in other embodiments. Thestarting Si wafer may have a passivated, low-reflection front surface,and a rear surface may be well-passivated and may contain either ana-Si:H heterojunction, a tunnel oxide junction or another layer thatcreates a strong inversion layer in the Si wafer.

In one embodiment, a solar cell with multilayer metallization isfabricated by applying a first patterned metal foil with a foildielectric insulation layer to a substrate, and then applying a secondmetal foil to the substrate. Referring to the process flow in FIG. 8, asilicon wafer is selected (S400) and is prepared by etching, cleaning,and texturing steps (S410) as described above. Antireflection andpassivation (S420) of the front side may be performed as describedabove. In the next step (S430), emitter or passivation layers may beapplied as discussed above.

Referring to process flow in FIG. 8 and device structure in FIG. 9, instep S440 a first metal foil assembly (475) may be applied by conductivebonding to the substrate (100). The first metal foil assembly (475) mayhave a metal foil layer (772), dielectric insulator (778), and/orbonding layers (835, 820). A foil assembly (475) may refer to a singlefoil or an assembly of a single foil with other components.

The first foil assembly (475) may be patterned and have patternedfeatures that produce openings (488) in the foil. The openings (488) inthe dielectric (778) may be aligned with high precision to the openingsin the metal (772). In some embodiments, the position of the edge of anopening in the metal (772) and edge of an opening in the dielectricinsulator (778) may be within 40 microns or less in a lateral direction,or within 10 microns or less in a lateral direction. Referring to FIG.10, in some embodiments, the dielectric insulator (778) may extend pastthe edge of the metal layer (772) opening, thereby producing an overhang(492). This may be desirable for avoiding possible contact or shortingbetween the first metal layer (772) and any subsequent layers.

The process of conductive bonding of foil to the rear of cell mayprovide sufficient electrical connection, such as where a substratecontains an emitter on the rear side. In some embodiments, laser firingthrough the foil assembly (475) may optionally be used to create pointdoping contacts or laser fired contacts (S450), such as for a substratethat has surface passivation on the rear of the cell. In some cases, thedoping material may come from the foil or a doping substance coated onthe foil. In some embodiments, even when an emitter exists on the rearside of the wafer, laser point firing can optionally be used to improvecontact to the emitter. A supplementary insulator layer may be appliedin step S460, with properties and function as described below.

Referring back to process flow in FIG. 8 and device structure in FIG. 9,laser base doping can be performed (S470) in order to produce basedoping contact points (364), which can be used to allow a second metalfoil assembly (480) that forms the second metal layer (782) to makecontact to the base (160). Laser doping can be performed as describedbelow.

In another embodiment, a solar cell with multilayer metallization isfabricated by applying a first patterned metal foil and then a secondmetal foil assembly with a foil and dielectric insulation layer.Referring to the process flow in FIG. 11, a silicon wafer is selected(S500) and is prepared by etching, cleaning, and texturing steps (S510)as described above. Antireflection and passivation (S520) are performedas described above. In step (S530) emitter or passivation layers areapplied as discussed above.

Referring to process flow in FIG. 11 and device structure in FIG. 12, instep S540 a first metal foil assembly (575) including a metal foil (772)and conductive bonding layer (820) is applied by conductive bonding tothe substrate. The foil assembly (575) may be patterned and havepattered features that produce openings (582) in the foil. As discussedpreviously, the metal foil assembly (575) may include other components,such as a bonding layer (820).

In some embodiments, the process of conductive bonding of foil assembly(575) to the rear of cell may provide sufficient electrical connection,such as for a substrate that contains an emitter on the rear side. Insome embodiments, laser firing through the foil may be used to createpoint doping contacts (S550), such as for a substrate that has surfacepassivation on the rear of the cell. In such cases, the doping materialmay come from the foil or a substance coated on the foil. It shall beunderstood that even when an emitter exists on the rear side of thewafer, laser point firing can be used to improve the contact to theemitter. A supplementary insulator layer may be applied in step S560,with properties and function as described further below.

A second metal foil assembly (580), including a dielectric (778) and ametal foil (782), can be bonded to the first metal foil assembly (575)in the next step (S570). The openings (586) in the dielectric insulator(778) may be aligned with high precision to the openings (582) in themetal foil assembly (575). In some embodiments, the position of the edgeof an opening (582) in the first metal foil assembly (575) and edge ofan opening (586) in the foil dielectric insulator (778) may be within 40microns or less in a lateral direction, or within 10 microns or less ina lateral direction. The centers of the dielectric openings (586) mayalign with the centers of the first metal foil openings (582). In someembodiments, the dielectric openings (586) may have smaller dimensions(e.g. diameter) than openings (582) to result in a slight overhang fromdielectric layer (778). This may be desirable for avoiding possiblecontact or shorting between the first metal foil and any subsequentlayers. Bonding layers (835) and conductive bonding layers (820) in theabove embodiments may be present on one or both of the surfaces to bebound.

In some embodiments, laser base doping can be performed (S570) in orderto produce base doping contact points (364) which can be used to allow asecond metal layer (782) to make contact to the base. Laser doping canbe performed as described below.

An exemplary embodiment is shown in FIG. 13. While the exemplaryexamples discussed herein may identify specific materials, doping types,processing steps and/or structural arrangements, it shall be understoodthat other materials, doping types, processing steps and/or structuralarrangements discussed in throughout specification may be substituted. Anonlimiting back-contact Al-foil heterojunction solar cell is shown forillustrative purposes only. The cell has a thin conductive bonding layer(820), such as indium or a tin-zinc solder, in contact with the 1^(st)Al or metal foil (772) to bond it to the rear conductive structure (240)on an a-Si:H i-p+ heterojunction (236) on the Si wafer (161). The 2^(nd)Al or metal foil (782) is coated with a layer of an n+ dopant (620)(such as a NiP alloy) and a dielectric layer (778). The laser firingthrough the 2^(nd) foil creates an n+ region (605) in the Si that allowsgood ohmic contact to the laser-transferred Al.

Another exemplary embodiment is shown in FIG. 14. This embodimentinvolves laser firing an array of p+ emitters (607) through a first Alor metal foil (772). Then a 2^(nd) Al or metal foil (782) coated with ann+ dopant source (620) and dielectric layer (778) may be applied, and n+regions (605) can be formed by laser doping through vias in the 1^(st)Al or metal foil (772). The rear surface may be passivated bya-Si:H/SiO_(x) (608), or other layers with a low density of fixedcharges so there is no significant band bending in the Si (or noinversion or accumulation layers).

A metal foil assembly bonded to a substrate may serve as a mask forprocesses on the substrate. As a non-limiting example, after step S440or step S540, the substrate will have regions covered and regions notcovered by a metal foil assembly. For example, in FIG. 9 when the metalfoil assembly (475) is bonded to substrate (100), regions under openings(488) will be not covered. Similarly, in FIG. 12, when metal foilassembly (575) is bonded to substrate (100), regions under openings(582) will be not covered. These substrates can be subjected toprocesses in which the uncovered regions can receive treatment(s), whilethe covered regions will be substantially unaffected because they arenot exposed. As a nonlimiting example, the uncovered regions may undergoprocessing such as wet treatments, wet etching, chemical vapordeposition, plasma enhanced chemical vapor deposition, physical vapordeposition, plasma etching, and reactive ion etching.

In one embodiment, the substrate (100) contains a rear conductivestructure (240, e.g. FIG. 9). After bonding of metal foil assembly(475), which simultaneous bonds the dielectric layer (778) and firstmetal foil (772) to the substrate, the resulting structure is processedin an etch process capable of attacking some or all of the conductivelayers that comprise layer (240). Similarly, in embodiments where thefirst metal foil (772) is bonded to the substrate prior to thedielectric layer (778) (e.g. FIG. 12), the first metal foil alone mayact as a mask for the etching process. Due to the masking effect of themetal foil assembly (475 or 575), only uncovered regions of the rearconductive structure (240) under the openings (488) will be partially orcompletely removed.

Patterned Foils Applied as a Multilayer Foil Assembly (MFA):

While the embodiments discussed above utilize processes where the layersare applied or exist prior to assembly on the cell as freestandingfoils, it shall be apparent that such embodiments may be modified forprocessing where the layers are applied as a MFA. Similarly, the MFAembodiments discussed below can also be modified for processing whereindividual layers are applied.

Embodiments using roll-to-roll processing with machine vision and laserprocessing are discussed herein. Supply rolls of metal foils anddielectric materials may supply materials for the roll-to-rollprocessing. In some embodiments, roll-to-roll processing comprisesmultiple lamination steps where layers are applied or joined together toform the layers of a multilayer foil assembly (MFA). In someembodiments, the lamination steps may merge a dielectric insulatinglayer to one or more of the metal foil layers to form the MFA. In someembodiments, the lamination steps may include other layers, such as, butnot limited to, bonding layers, dopant source layers or layers forpromoting the formation of metal silicides (such as NiSi) forlow-resistance contacts. In some embodiments, the roll-to-rollprocessing may include steps to form patterned openings or an array ofholes at desired regions in the foil assembly, such as by drilling ofvia arrays. It shall be understood that these opening formation stepsmay be performed on the layers-of-interest at any time prior tolamination to a layer that does not require the openings. Further, theopening formation steps may also be performed after layers-of-interestrequiring the same openings are laminated together. In some embodiments,it may be desirable to perform opening formation steps to providefiducial holes that may be utilized to aid aligning layer(s). Thus,opening formation step(s) may be performed on a metal foil layer priorto lamination to another layer of material; after lamination of a metalfoil layer to an insulator dielectric layer and/or a bonding layer, butbefore lamination to a second metal foil layer; and/or after laminationof a second metal foil layer to an insulator dielectric layer and firstmetal foil layer. It shall be apparent from the following discussionthat the desired combination of above noted opening formation step(s) isdependent on the rear contact structure desired. In some embodiments,the roll-to-roll processing may involve high-speed dopant printing (e.g.inkjet, aerosol jet, laser transfer printing, etc.), such as in viascreated by an opening formation step. In some embodiments, thehigh-speed dopant printing may include depositing an array of printheads or nozzles and multiple dopant materials. Once the multilayer foilassembly is formed by the roll-to-roll process, it can be bonded to asilicon wafer during the laser firing/doping of a back-contact solarcell through the vias formed in the opening formation step(s) to formlaser fired contacts for a first metal foil, laser fired contacts for asecond metal foil, or both.

In some embodiments, the solar cell fabrication involves three separateprocesses where wafer cleaning, texturing and passivation (includingheterojunction formation or tunnel oxide formation) are performed in oneprocess, while the roll-to-roll processing of the multilayer foilassembly can be performed simultaneously in a separate operation. Thefoil assembly may then be laser doped and bonded to the passivated waferin a final solar cell processing step.

Referring to process flow in FIG. 15 and device in FIG. 16, a siliconwafer (S700) may be selected and prepared by etching, cleaning, andtexturing steps (S710) as described above. Antireflection andpassivation (S720) are performed as described above. In the next step(S730), emitter or passivation layers are applied as discussed above.

In some embodiments, a multilayer foil assembly (760) may be prepared ina high volume process external to the solar cell fabrication line orseparately from the substrate prior to application on the substrate. Themultilayer foil assembly may contain a first metal foil (772), a secondmetal foil (782), and a dielectric film (778) disposed between the twoin order to prevent electrical contact between the first and secondmetal foils. One or more layers of the multilayer foil assembly may bepatterned as described below to produce openings, such as to produceopenings (914) in the metal foil (772) and dielectric (778)corresponding to doped regions for the base, which allow the upper foil(782) to make contact with substrate. The openings in the dielectricinsulator (778) may be aligned with high precision to the openings inthe first metal foil (772). In some embodiments, the position of theedge of an opening in the metal foil (772) and edge of an opening in thedielectric insulator (778) may be within 40 microns or less in a lateraldirection, or within 10 microns or less in a lateral direction. In someembodiments, processes may be employed to allow dielectric insulator(778) to extend past the edges of the first metal foil (772), therebyproducing an overhang (759). This overhang (759) may be desirable foravoiding possible contact or shorting between the first metal foil (772)and any subsequent layers. As in other embodiments, laser processing maybe performed (S740) to form laser fired contacts that are electricallycoupled to the upper foil (782).

In some embodiments, there may be openings (764) in the second foil(782) corresponding to doped regions for an emitter to allow a laserbeam or other process to address the first foil (772) for the purpose oflaser doping the substrate with the first foil (S750).

The multilayer foil assembly (760) may be conductively bonded to thesubstrate, establishing a connection of the lower metal foil (772) toregions of the substrate. In some embodiments, conductive bonding mayinclude a conductive bonding layer (820) and process as described below.

One nonlimiting embodiment of a solar cell with multilayer metallizationyielding a back-contact, p-type Si heterojunction solar cell is shown inFIG. 17 including a multilayer foil assembly (760), with a process forproducing the multilayer foil assembly shown in FIG. 18. As anonlimiting example, a back-contact Al-foil heterojunction (HJ) solarcell on a p-type Si wafer is discussed, but it shall be understood thatother embodiments may vary the materials involved and/or steps performedaccording to the multilayer foil assembly desired. Similarly, it shallbe understood that further examples discussed herein may also vary thematerials involved and/or steps performed. For an Al-foil HJ solar cell,a first metal foil (772), supplied from source (770), may be laminatedto a dielectric layer (778), supplied from source (775), by a joiningprocess (785) and then both layers perforated, for example, by drilling(792) an array of small holes or vias at desired locations. The viascould be formed using an array of lasers or by mechanical perforation.The perforated, laminated metal/dielectric layers are then laminated toa second metal foil (782), supplied from source (780), by a joiningprocess (786). In some embodiments, machine vision may be used inconjunction with laser drilling to assist proper alignment. As anonlimiting example, machine vision may be used to direct another laserto drill fiducial holes (794) through the laminated sheet to assist inthe subsequent alignment of a p+ doping laser with the array of vias inthe 1^(st) metal foil. The resulting roll form multilayer foil assembly(787) is optionally taken up on roller (790) for shipping and storage.

Subsequently, multilayer foil assembly sections may be cut and placed incontact with a Si HJ layer on a Si wafer (100, FIG. 17), and a laserforms p+ base contacts while locally disrupting the Si HJ contact so asto inhibit shunting.

In one particular nonlimiting embodiment employing the process of FIG.18, a first Al foil is laminated to a thin layer of PET, and a laserthen drills vias through the laminated Al/PET layers. A second Al foilis laminated to the perforated Al/PET layers, and another laser drillsfiducial holes through the foil assembly to align the p+ contacts withthe vias. A thin In or Sn—Zn solder bonding layer (820) could be eithersputtered onto the surface of the rear conductive structure (240) oronto the surface of the bottom multilayer foil assembly (760) (as partof the multilayer foil assembly formation process). Another option is tospray a conductive adhesive, such as an Ag, Cu or Ni-based adhesive, tobond the rear conductive structure (240) to the 1^(st) Al foil (772)with a low contact resistance.

One embodiment of a solar cell with multilayer metallization yielding aback-contact, n-type Si HJ solar cell is shown in FIG. 19 including amultilayer foil assembly (760), and a process for producing themultilayer foil assembly (760) shown in FIG. 20. As a nonlimitingexample, the following process may be used for a back-contact Si HJsolar cell on an n-type Si wafer.

Referring to FIG. 20, as before a first metal foil (772) is laminated toan insulating layer (778) (e.g., PET) and a laser is used to drill anarray of vias. After laminating a 2^(nd) metal foil (782) to theperforated PET/metal foil, machine vision may direct a printer (e.g.with multiple print heads or nozzles) (796) to deposit an n+ dopant inkthrough the vias in the 1^(st) metal foil (772) and insulating layer(778) onto the exposed regions of the 2^(nd) metal foil. In anotherembodiments, the exposed regions of the 2^(nd) metal foil may beelectroplated with an n+ dopant source (such as a nickel phosphorusalloy with 12 wt. % phosphorus). In this case, multilayer foil assembly(787) can be placed in an electrochemical-plating bath with the metalfoils biased so that the n+ dopant is deposited only in the exposedregions of the 2^(nd) metal foil. A vision system is then used to directa laser (794) to drill fiducial holes through the foil assembly, such asto aid alignment as discussed previously.

In yet another embodiment, the vias or openings in multilayer foilassembly (787) may be filled with a dopant source using a bathcontaining the dopant source in a liquid or fluid paste form.Subsequently, a squeegee may be utilized, such as in a roll-to-rolloperation, to removes the excess dopant source material from the surfaceof the multilayer foil assembly (787) or from everywhere on themultilayer foil except the vias. The foil assembly could then passthrough a heated stage where the volatile components in the dopantsource in the vias are driven off. The dopant source may be selected toyield a doping characteristic opposite to the doping that would arise ifthe foil itself is the dopant, as discussed below.

Once the multilayer foil assembly (787) is complete, an section of themultilayer foil assembly section may be cut out and placed in contactwith a Si HJ layer on a wafer (100, FIG. 19), and the vision system usesthe fiducial holes to direct a laser to fire through the 2^(nd) Al foil(782) and n+ regions (620) in the vias to form the n+ base contacts(605) in the Si wafer.

In one particular nonlimiting embodiment, a laser drills vias throughthe PET-laminated 1 ^(st) Al foil, and after laminating to the 2^(nd) Alfoil, a high-speed printer is used to deposit an n+ dopant source (suchas a commercial phosphorus dopant ink) on the exposed Al of the 2^(nd)foil in the vias as shown in FIG. 20.

In this embodiment, an n+ dopant source (620) is deposited on the 2^(nd)Al foil (782) through the vias in the 1^(st) Al foil (772) and theinsulating layer (e.g. PET, 778) as shown in FIG. 19. A laser pulsefired through both the 2^(nd) Al foil (782) and the n+ dopant source(620) will form an n+ region (605) in the silicon and will also deformand melt the Al foil into the n+ region forming an ohmic base contact.

One embodiment of a solar cell with multilayer metallization yielding aback-point-contact Si solar cell is shown in FIGS. 21 and 22, includinga multilayer foil assembly (760), formed with the previously discussedprocess for producing the multilayer foil assembly (760) shown in FIG.19. The following process may be used for a back-contact Al-foil solarcell where both p+ and n+ contacts are formed by laser firing/doping.

Referring to FIG. 23, as before, a 1^(st) metal foil (772) is laminatedto an insulating layer (e.g., PET, 778) and a laser or patterningoperation (792) is used to drill a 1^(st) array of vias. Another laseror patterning operation (798) drills a 2^(nd) array of vias in a secondmetal foil (782), and the foils are registered so that the 2^(nd) arraysof vias are in the proper location after lamination.

A machine vision system may direct a printer (796) to deposit an n+dopant ink (620) through the vias in the 1^(st) metal foil/PET (772/778)onto the exposed regions of the 2^(nd) metal foil (782). A vision systemis then used to direct a laser (794) to drill fiducial holes (765)through the foil assembly.

Referring to FIG. 21, a multilayer foil assembly (760) section is cutout and placed in contact with a passivated rear surface of a Si wafer,and a vision system directs a laser to fire the 2^(nd) Al foil (782) andthe n+ dopant source (620) through the 1^(st) array of vias to form then+ base contacts (605) in the Si wafer. Another laser is directed tofire through the 2^(nd) array of vias in the 2^(nd) Al foil (782)ablating the PET (778) and firing the 1^(st) Al foil (772) into the Siwafer to form p+ contacts (745).

As before and as shown in FIG. 23, a laser drills a 1^(st) array of viasin the 1^(st) Al foil/PET laminate (772/778). Another laser drills a2^(nd) array of vias in the 2^(nd) Al foil (782) for subsequentformation of the p+ contacts (745). The foils are registered to eachother to assure proper placement of the via arrays. An n+ dopant source(620) is deposited through the 1^(st) array of vias onto the 2^(nd) Alfoil (782).

In this embodiment, an multilayer foil assembly (760) is placed on therear surface of an n-type Si wafer (161) passivated, for example, bySiO_(x):H/Al₂O₃ layers (634) as shown in FIG. 21.

A laser forms the p+ emitter contacts (745) through the 2^(nd) array ofvias by optionally ablating the dielectric (e.g., PET) (778) and firingthe 1^(st) Al foil (772) into the n-type wafer (161). In someembodiments, the dielectric (778) is minimally affected by the laserpulse and partially or completely remains in opening (764) aftercontacts (745) are made (See e.g. FIG. 16). Another laser forms the n+base contacts (605) by firing the 2^(nd) Al foil (782), and thedepositing n+ dopant source (620) through the 1^(st) array of vias. Thisembodiment shows an inversion layer (763) induced by the Al₂O₃, but itcould be created by a diffused layer, a tunnel oxide or aheterojunction.

In one embodiment, referring to FIG. 22, a back-point-contact Al-foilsolar cell (p-type Si) is shown. In this embodiment, an multilayer foilassembly similar to the above (760, FIG. 19) is placed on the rearsurface of a p-type Si wafer (162) passivated by a layer structure suchas a-Si:H/SiOx (608), which has a low density of fixed charges so thereis no significant band bending in the Si (or no inversion oraccumulation layers). A laser forms the p+ base contacts (745) throughthe 2^(nd) array of vias by ablating the PET (778) and firing the 1^(st)Al foil (772) into the p-type wafer (162). Another laser forms the n+emitter contacts (637) by firing the 2^(nd) Al foil (782) and thedeposited n+ dopant source (620) through the 1^(st) array of vias.

One embodiment of a solar cell with multilayer metallization yielding aback-point-contact Si solar cell with the formation of a nickel silicidecontacting layer (644) is shown in FIG. 24, including a multilayer foilassembly (760), and a process for producing the multilayer foil assembly(760) is shown in FIG. 25. This embodiment includes using an inkcontaining a material useful to reduce contact resistance, such as smallnickel particles (≤1 μm in size) to form a nickel silicide (e.g. NiSi)at the base point contact. In structures where both p+ and n+ contactsare made by laser firing through the 1^(st) and 2^(nd) Al foils (e.g.see FIGS. 21, 22 and 24), the laser firing process may adequately bondthe multilayer foil assembly (760) to the substrate (100) so that abonding layer (820) (e.g. such as shown in FIG. 19) is not required.

Any substance capable of producing low specific contact resistancecontacts to silicon or other semiconductors may be utilized. Variousmetals may be incorporated at or near the silicon surface to produce lowcontact resistance. These metals may include nickel, palladium,titanium, platinum, molybdenum, tungsten, or the like. The metals may besupplied as deposited coatings on the silicon substrate or on a foilassembly that is being applied to the substrate. The substance may besupplied as an ink containing the metal, either in the form of achemical precursor to the metal, particles of the metal, or particles ofa compound containing the metal. Particularly useful particles includesilicon compounds of the metal, such as silicides.

The process shown in FIG. 25 is similar to FIG. 23. As such, discussionof the common steps is not repeated for the sake of brevity. In thisprocess for FIG. 25, a Ni ink, for example, is deposited (797) on theexposed regions of the 2^(nd) Al foil in the 1^(st) array of vias, andthen n+ dopant source is deposited (796) in the same vias. The Ni inkmay be deposited in the 1^(st) array of vias before depositing the n+dopant source in those vias.

Since dopants such as phosphorus appear to diffuse more rapidly inmolten silicon than nickel, other embodiments might use an inkcontaining a mixture of a phosphorus dopant source and Ni particles(e.g. on the order of a micron or less in size) in combination withlaser conditions that promote the formation of a nickel silicide layeron top of an n+ region in the n-type Si wafer, as well as a lowresistance contact (605) of the Al from the 2^(nd) Al foil to the nickelsilicide layer. If one uses an ink containing both the dopant source andthe Ni particles, then only one deposition step is required. In otherembodiments, an ink containing small particles of a nickel phosphorusalloy may be utilized. The phosphorous content may be equal to or lessthan 50%, or equal to or less than 20%.

Referring to FIG. 24, a multilayer foil assembly (760) made by theprocess shown in FIG. 25, where both a Ni ink (640) and a n+ dopantsource (620) are deposited on the exposed 2^(nd) Al foil (782) in the1^(st) array of vias, is applied to a substrate 100.

In this embodiment, the laser forming the base contacts (605) fires the2^(nd) Al foil (782), the Ni ink (640) and the n+ dopant source (620)through the 1^(st) array of vias into the n-type Si wafer. The laserconditions are chosen so that a nickel silicide layer (644) forms on topof the n+ region in the Si and is contacted by Al from the 2^(nd) Alfoil (782).

Emitter forming layers on the rear of the substrate, such asheterojunction layers (235), passivation layers or other layers, maycreate induced emitters (induced inversion layers) on the substrate thathave moderate to high conductivity. Referring to FIG. 19 as anon-limiting example of the general phenomenon, if heterojunction layers(235) make partial contact with base doped region (605), the cell maydevelop a short circuit or shunt in which useful current is wasted bythis undesirable conduction. Therefore, it may be desirable to employmethods that disrupt these layers in the vicinity of the base contacts(for example, 610 in FIGS. 13, 17, 19, 21, and 22). This may beaccomplished by selecting a laser doping pulse with sufficient length,energy, and/or intensity to create a high temperature region around eachn+ doped region that locally disrupts the surface layers. The disruptionmay include partial or complete removal of surface layers, such as theheterojunction layers (235), thereby leaving a gap or disconnect betweenthe heterojunction layers and base doped region (605) that preventsshort circuiting or shunting. Alternatively, the disruption may includesufficient heating or light exposure to change the electrical propertiesof the surface layers.

The dopant source may be applied as a coated dopant material to a metalfoil. The coated dopant source may be a uniform or patterned coatingapplied on the side of the metal foil facing the substrate. In someembodiments, the metal foil may contain atom(s) that itself causesdoping. As a nonlimiting example, the metal foil may contain aluminum,which can create p-type doping in silicon. The coated dopant source maycontain atom(s) which produces a dopant polarity opposite to thepolarity of the dopant atom in the metal foil. As a nonlimiting example,an aluminum metal foil may be coated with a phosphorous doping source.In such cases, it has been unexpectedly found that while the foilcontains a large quantity of the p-type dopant (e.g. aluminum), theresulting contact, which achieved when laser firing through the aluminumfoil with the doping source, is n-type (e.g. phosphorous). Not wishingto be bound by theory, it is postulated that during laser firing, meltedsilicon is produced in the region of the laser fire, and that thephosphorous n-type dopant has a higher diffusion coefficient than thealuminum p-type dopant.

The dopant source may be any material containing the desired dopant.Nonlimiting examples of dopant sources may include phosphosilicateglasses, aluminum phosphates, phosphoric acid and phosphoric acidderivatives, organophosphorus compounds, and inert organic or inorganicmatrices containing phosphorous compounds.

Incorporation into Modules:

FIG. 34 shows an illustrative embodiment of a solar cell system or solarcells (900, 901, 902) (e.g. Al-foil cell) connected together in seriesat interconnection (908) (e.g. by laser welding) to form one string in aphotovoltaic (PV) module. In one embodiment, the connection from onecell to another is a direct connection, meaning a metal foil layer fromone cell is electrically connected to a metal foil layer of another cellwithout intervening wires or other conductive means. In someembodiments, a first metal foil of a first solar cell (900), which isalso electrically coupled to the first doping region of the first solarcell, is directly connected to a second metal foil of a second solarcell (901), which is also electrically coupled to second doping regionsof the second solar cell. Further, a second metal foil of the firstsolar cell (900), which is also electrically coupled to second dopingregions of the first solar cell, is further connected to a first metalfoil of a third solar cell (902), which is also electrically coupled tofirst doping regions of the third solar cell. In this manner, the cellsare connected in series to produce a string of cells. Further, thismechanism of connecting the cells in series allows the cells to bearranged side to side. The positive and negative ends of each string arelaser welded to a wire, bar, or sting connector 910 (e.g. Alwire/bar/connector) that electrically connects the strings together. Thestring connectors 910 run to a junction box where they connect to theexternal wiring of the PV system. In some embodiments, the Al foils canbe welded in series to form a string in a PV module by using ultrasonicwelding, micro TIG (tungsten inert gas) welding or soldering using asolder such as tin-zinc alloy (e.g. 91% Sn, 9% Zn).

General Description of Laser Processing:

Laser doping, such as for forming base contacts or the like, may beperformed by processes including laser firing, laser doping, lasertransfer doping, and/or gas immersion laser doping (GILD). The variouslaser processing techniques discussed below can be utilized for any ofthe embodiments discussed herein.

In some embodiments of a laser doping or laser firing process, a dopantsource may be applied to the silicon substrate and/or driven into thesilicon during the laser pulse while in the laser firing process. Insome embodiments of the laser doping or laser firing process, a metaldopant, metal layer, or metal dopant on a metal layer may be applied tothe substrate and/or driven into the substrate during a laser pulse. Insome embodiments, the dopant layer may be applied by physical vapordeposition techniques, including evaporation and sputtering. In someembodiments, the dopant layer may be applied by chemical vapordeposition techniques. In some embodiments, the dopant layer may also beapplied by liquid deposition techniques, including screen printing,squeegee application of a liquid or fluid paste, spin coating, beadcoating, electroplating, or inkjet printing. The liquid dopant can besupplied at a solution or dispersion or slurry.

In some embodiments of the laser transfer doping process, the dopant issupplied from an additional donor substrate. This approach utilizes arapid interaction between a laser and a non-transparent thin source filmdeposited on a transparent plate or donor substrate (e.g. glass orquartz), which is placed in close proximity (e.g. about several micronsor less) to a substrate. The dopant source may be applied to the donorsubstrate by any of the approached mentioned above. The donor substratemay also include thin flexible glass and/or polymer films and othermaterials transparent to the laser radiation.

The donor or transfer substrate of the laser transfer system can becoated with multiple layers depending on the application. As anonlimiting example, the laser transfer substrate may be first coatedwith a thin easily evaporated material (e.g. a-Si:H) to act as a releaselayer for a refractory material (e.g. Mo) or a transparent material(e.g. SiO₂) deposited on the a-Si:H. Another example involves firstdepositing a layer of Ni on the laser transfer substrate followed by alayer of Sb so that the laser will transfer Sb for n⁺ doping and Ni fora low-resistance nickel silicide contact.

The laser transfer system can utilize multiple pulses in addition totemporally shaped pulses. As a nonlimiting example, the first pulsecould comprise a first section of relatively high energy density (e.g.˜1 j/cm² or greater) over 10 ns, and then a slowly decreasing sectionwhere the energy density decreases from equal to or betweenapproximately 0.7 to 0.1 J/cm² over 400 ns. A second pulse to the samelocation might then be applied approximately 10 μs later (100 kHzrepetition rate) with an energy density ramping up to ˜0.3 J/cm² over 10ns, and then slowly decreasing to 0.05 J/cm² over 500 ns to furtheranneal the treated region. The wavelength of the laser beam can be inthe IR (e.g. 1064 nm) for most applications, but a laser beam operatingin the green (532 nm) can also be used and will more effectively heatjust the top few μm of an exposed Si surface. The IR beam will initiallyheat the Si wafer to a depth of a few hundred μm, but as the laserrapidly heats up the Si locally, the absorption coefficient in the IRincreases rapidly and the heating becomes localized near the surfaceregion.

In some embodiments, a Gas Immersion Laser Doping (GILD) process may beutilized, where the dopant is supplied in vapor form to a chamber abovethe substrate such that the dopant vapor is in gaseous communicationwith the substrate. The gaseous dopant or a byproduct is incorporatedinto the substrate during the laser pulse. Gaseous source(s) can be anymaterial containing a dopant atom with sufficient volatility, including,but not limited to, POCl₃, PCl₃, PH₃, BH₃, B₂H₆, arsine, andtrimethylaluminum.

The laser process may utilize spatially and temporally shaped laserbeams. In some embodiments, the systems or methods discussed herein mayhave the following elements: (1) supply of dopants by a laser process;(2) dopants supplied by the laser process to avoid heating of the wafersto perform dopant diffusion; and/or (3) a back contact (IBC) cell. Theuse of line beams is a particularly attractive way to make an IBC cellsince the electrodes of the IBC are thin lines, and thus can bepatterned with single or reduced number of laser pulse exposures. Insome embodiments, the combination of (1)-(3) above may be utilized withline and/or temporal shaping. In some embodiments, the laser beam can bespatially shaped into a narrow line-shaped laser beam or into an arrayof very small diameter Gaussian laser beams (e.g. <20 μm or <10 μm).Line-shaped laser beams with widths <10 μm exhibit little laser-induceddamage, while suitable circular Gaussian laser beams (e.g. withdiameters of ˜30-130 μm) exhibit microcracks and dislocations. Smalldiameter (<20 μm or <10 μm) Gaussian laser beams are also less likely toexhibit extended defects, such as microcracks and dislocations due tothe fact that only a very small region of Si is melted andrecrystallized.

The temporal pulse shape can be selected for the purposes of lasertransfer of material, laser ablation or disruption of dielectricpassivation layers, laser melting of selected localized regions of theSi wafer, laser doping of the melted Si regions with the appropriatedopant atoms, laser-induced formation of metal silicides forlow-resistance contacts, laser firing of contacting metals through thedielectric passivation layers and/or laser annealing of the localizedtreated regions on the Si wafer. Generally, laser transfer of materialrequires relatively short pulses (e.g. few ns to few tens of ns), whilelaser annealing requires relatively long pulses (e.g. 0.1 μs to severalmilliseconds). The pulse duration for laser doping will depend on thedopant depth desired and can vary from tens of ns to hundreds of ns. Insome embodiments, the dopant penetration depths are equal to or between0.02 μm to 1 μm, or equal to or between 0.1 μm to 0.5 μm. As anonlimiting example, a laser process which combines laser transfer,disruption of the dielectric passivation, melting, doping and annealingof the Si in a localized region might employ a line-shaped beam (e.g. 8μm wide and 1 cm long) with the following temporally shaping: the pulsestarts with an energy density of ˜1 J/cm² over several ns to transferthe dopant material (e.g. Al) to the substrate (e.g. Si surface) anddisrupt the dielectric passivation, if present (e.g. 5 nm of ALDAl₂O₃/90 nm of PECVD SiO_(x) on the rear surface); the energy densitythen falls to ˜0.5 J/cm² over ˜50 ns to locally melt the substratesurface and diffuse in the dopant; and then the pulse energy densitydecreases from 0.5 to 0.1 J/cm² over ˜400 ns to anneal the localizedregion of substrate surface.

As nonlimiting examples, dopant materials may be any suitable n- orp-type material, Al, Sb, Group III or V element, or the like. In thelaser transfer process, the dopant atom is introduced on a donorsubstrate containing or coated with a dopant material including thedonor atom. In a laser firing or doping process, the similar dopant isincluded as a metal or dopant material, respectively, on the substrateto be fired. The dopant material may be a pure form of the dopant, suchas but not limited to coatings of the group III or group V atoms.Alternatively, the dopant material may be a compound containing thedopant, such as but not limited to an oxide, nitride, or chalcogenide ofthe donor. The dopant material may also be composed of an alloy or ahost material containing the dopant, such as hydrogenated amorphoussilicon heavily doped with the dopant. Concentration of the dopant inthe host material may be equal to or greater than 0.5%, or equal to orgreater than 2%.

General Bonding and Conductive Bonding:

The various bonding techniques discussed below can be utilized for anyof the embodiments discussed herein. An example of general bonding,bonding layer(s) (e.g. 820,835 in FIG. 9) serves to bond two discretelayers together to achieve a mechanical connection.

The bonding layer may include an adhesive, such as a pressure sensitiveadhesive. Nonlimiting examples of pressure sensitive adhesives mayinclude acrylics, rubbers, ethylene-vinyl acetates, nitriles, silicones,or styrene block copolymers. The adhesive layer can be applied atvarious stages of production, including during the formation of the foilfilm or during the solar cell fabrication process. Surfaces on which thepressure sensitive adhesive exist or attach to may be treated to promoteadhesion. Nonlimiting examples of such treatments may include cleaning,solvent cleaning, chemical treatment, ozone treatment, corona discharge,and/or plasma treatments.

In some embodiments, the bonding layer may be or may include a curingadhesive that acts to bond the surfaces. A curing adhesive is anadhesive that has low tack or is liquid in its native state, but is thenin modified to produce adhesive behavior before, during, or after thesubstrates to be bonded are attached. The curing adhesive can be curedby any suitable technique including exposure to heat, moisture,activating compounds, and/or light. Nonlimiting examples of curingadhesives include acrylics, polyesters, urethanes, polyol-urethanes,epoxies, polyimides, and cyanoacrylates.

In some embodiments, the bonding layer may be or may include adhesivepolymers in which bonding is achieved by exposing the joined surfaces totemperatures sufficient to allow the polymer to flow or melt.Nonlimiting examples of such polymers include polyvinyl alcohols,polyvinylbutyrals and ethylene-vinyl acetates. In some embodiments, thebonding layer may be or may include laminating adhesives, including, butnot limited to, polyurethanes.

The bonding layer may be applied by any suitable process. Nonlimitingexamples of such application processes include bead coating, spraycoating, aerosol vapor coating, and extrusion. In some embodiments, thebonding layer may be applied in a patterned fashion so as to be onlypresent in certain locations. Nonlimiting examples of patterningapplication methods include inkjet printing, gravure printing, andscreen printing.

The bonding layer can have any thickness suitable to achieve usefuladhesion. As a nonlimiting example, the bonding layer thickness may bein the range of equal to or between 1 to 50 microns, or equal to orbetween 3 to 30 microns. The bonding action may be aided by mechanicalpressure applied to form the bond. Nonlimiting examples of suitablemechanical pressure may include pressure applied by rollers, uniformpressure applied by a plate or mold, and air pressure.

Conductive bonding is a variation of bonding in which, in addition to amechanical bond, the surfaces being joined together achieve suitableelectrical contact. In some embodiments, electrical contact may includea specific resistance between layers of equal to or less than 1 ohm-cm²,or equal to or less than 0.2 ohm-cm².

In some embodiments, conductive bonding layers may exist on one or bothof the surfaces to be placed in contact for bonding. As a nonlimitingexample, in FIG. 12, a bonding layer (820) may be present on metal layer(772) and another bonding layer may be present on layer (240). Theconductive bonding layer may comprise any of the material described forgeneral bonding with the incorporation of a conductive material.Nonlimiting example of suitable conductive materials may include metalparticles, carbon particles, and/or semiconductor particles. In someembodiments, conductive particles may be equal to or less than 30microns in effective circular diameter, or equal to or less than 10microns in effective circular diameter. In some embodiments, the bondinglayer (820) may comprise metal particles in an adhesive matrix, wherethe metal particles include silver, copper, nickel, or the like. In yetanother embodiment, the bonding layer (820) may comprise graphiteparticles in an adhesive matrix.

The conductive bonding layer may include a metal or alloy that produceselectrical contact through methods including heating and/or pressure.Nonlimiting examples of suitably metals or alloys may include indium,solders, and/or tin. Metals include metals or alloys that melt at equalto or less than 350 C, or equal to or less than 300 C. Tin-zinc alloysmay be used to solder aluminum to aluminum or copper at relatively lowtemperatures (e.g. equal to or between 200° C. to 260° C.). In someembodiments, surface treatment agents such as flux may be used toimprove metal adhesion and wetting.

Referring to FIG. 32, a surface of a metal foil (772) may provide atexture (840) that allows for improved electrical connection using abonding layer (835). Referring to FIG. 33, when metal foil assembly(475) is brought in contact with substrate (100), texture (840) (shownin FIG. 32) may allow penetration (842) of the metal through the bondinglayer (835) to produce good contact to the rear conductive structure(240). In some embodiments, the metal foil texture (840) may includetextures formed by chemical etching and mechanical abrasion. In someembodiments, the texture (840) may be formed by an embossing process. Insome embodiments, the substrate surface at layer (240) may be texturedand may be joined with a metal foil (772) with or without a texture. Insome embodiments, this texturing may be achieved by depositing aconductive material on the substrate or the foil to form protrusions onthe surface. The features of the texture may be equal to or less than0.5 mm in diameter and/or height, or equal to or less than 0.1 mm indiameter and/or height.

Bonding or conductive bonding layers may be coated uniformly on thesurfaces of a metal foil or metal foil assembly. Alternatively, bondinglayers may be patterned for reasons including minimizing the usage ofthe bonding material or preventing unnecessary shunting or electricalconduction.

Supplementary Insulation Layer:

In some embodiments, a supplementary insulating layer can be thinnerthan the dielectric insulator layer (e.g. 350, FIG. 6). In someembodiments, the supplementary or secondary insulating layer can beequal to or between 1 micron to 20 microns thick, or equal to or between2 microns to 30 microns thick. In some embodiments, the secondaryinsulating layer may include epoxies, acrylics, polyesters, polyvinylbutyrals, and/or polyvinyl alcohols. In some embodiments, the secondaryinsulating layer may be coated with a suitable solvent, including water,cyclohexanone, propylene glycol monomethyl ether, acetone, methylisobutyl ketone, and methyl ethyl ketone. In some embodiments, thesecondary insulating layer may contain a colorant, dye, or pigment thatabsorbs at the wavelength of the laser that will be used for doping,which allows the secondary insulation layer to be more easily heated andablated for proper doping and contact. Nonlimiting examples of suitabledyes include dyes absorbing at wavelengths equal to or betweenapproximately 1064 nm to 355 nm, which includes wavelengths forinfrared, green, and UV lasers. Nonlimiting examples of pigments includecarbon black. In some embodiments, carbon black may be present asparticles equal to or less than 5 microns in size, or equal to or lessthan 1 micron in size. The carbon concentration can be maintained at avalue lower than the percolation threshold so that the secondaryinsulation layer has high resistance. In some embodiments, the carbonblack concentration may be equal to or less than 10%, or equal to orless than 3% by weight relative to the binder polymer.

Foil Patterning:

Referring to FIG. 9 as a nonlimiting example, a metal foil assembly maybe patterned by any suitable method. In some embodiments, the metal foilassembly may be patterned in a roll-to-roll process, or as sheet stockin a batch process. The patterning process may include laser ablation orlaser cutting, mechanical patterning such as perforation, dry phasepatterning and/or chemical processes such as etching. The laserprocessing may include using a laser with a short pulse, such as a laserwith a pulse length equal to or less than 100 ns, or equal to or lessthan 1 ns. Dry phase pattering may use a groove pattern on a wheel orplate to press a relief pattern on a material that may include multiplelayers. A milling wheel then mechanically removes portions of theprotruding pattern. This method can independently pattern a subset ofthe layers from an existing multilayer foil assembly.

Foil Application Processes:

Referring to a nonlimiting example in FIG. 26, a general multilayer foilstructure (e.g. 760) may be applied to a surface of substrate (e.g.100). The multilayer foil structure may be created by any of the methodsdiscussed herein, including application of one or more multilayer foilassemblies and/or application of individual foil elements. In someembodiments, a gap (e.g. 812) may exist between the substrate (e.g. 100)and the second foil (e.g. 782), and improved contact between thesubstrate or the laser doped region (e.g. 364) and the foil may beobtained if foil (782) approaches the substrate. The gap may exist neara general opening (914) in the multilayer foil assembly, where thegeneral opening is an opening made by any of the methods described aboveand is for at least the purpose of allowing contact of the upper foil(782) to the substrate.

In one embodiment, a sequence of one or more laser pulses may be used tosoften or melt the foil (e.g. 782) so that it makes contact to thesubstrate (e.g. 100). Such laser pulses may be in the range of equal toor between 1 μs to 10 ms. Such laser pulses may be focused to provideheating over relatively large areas, such as, but not limited to, areasequal to or greater than 2500 μm².

In another embodiment (FIG. 27), a predetermined force (830) may be usedto cause the second foil (782) to approach and contact the substrate bylocally deforming (837) the second foil. With sufficient bonding forceor pressure (830) a contact region (832) can form between the laserdoped region (364), second foil (782), and/or bonding layer (820).

In some embodiments, a conductive bonding layer (820), such as aconductive adhesive or solder, may be present on one or more of the foilelements such than when the bonding force or pressure (830) is appliedconductive contact is made between the second foil (782) and thesubstrate. A doped region (364) may already exist on the substrate andmay be partially or completely in contacted with the second foil (782)after application of pressure (830). The doped layer (364) could beformed by localized processing such as laser doping or ion implantationand laser annealing. In some embodiments, an additional activation stepfor the conductive bonding layer as previously described above may beapplied during or near the timeframe of the application of force orpressure (830), such as a curing step to activate and bond theconductive adhesive or to melt solder. In the case of solder, a fluxingagent may be applied locally to form a low resistance adherent bond.

In some embodiments, the specified force (830) may be mechanicalpressure, such as a roller, compliant plate, press or the like. In someembodiments, the specified force (830) may be a gas pressure. As anonlimiting example, gas pressure can be accomplished by applying amembrane or a foil over the sample and producing a vacuum under themembrane or foil. This may allow pressures up to one atmosphere to acton the sample. In some embodiments, pressures greater than 1 atmospherecan be applied from above the sample alone or in conjunction with othermethods discussed herein. In some embodiments, an array of gas jets maybe used to apply pressure or additional pressure from above the sample.In some embodiments, total pressure on the sample by any method may bein the range equal to or between 0.25 to 20 atmospheres, or in the rangeequal to or between 0.5 to 10 atmospheres.

Referring to FIGS. 28 and 29, in some embodiments, a conductive bondinglayer (820) may exist on the substrate (or alternatively on second foil782 or both) prior to application of the first metal foil. FIG. 28 showsthat device structure prior to application of a bonding force, and FIG.29 shows the device structure after application of a bonding force(830). In some embodiments, the same bonding agent may be used forconductive bonding of both the emitter and the base contact. As aresult, in some embodiments, the conductive bonding layer may have acomposition such that it exhibits high conductivity in a verticaldirection through the film, but a low conductivity in lateral directionthrough the film. This property may enable good electrical connectionfrom the second foil (782) to the substrate, but low conductivity fromthe doped region (364) to the regions on the first foil (772) that mayproduce a short or shunt in the solar cell. Conductive bonding layers(820) for this application may include anisotropic conductive films oranisotropic conductive adhesives.

In the embodiments of FIGS. 28 through 29, part of the laser doped basecontact (364) may exist on the substrate prior to application of a partof the metal foil (772, 782) or metal foil assembly (760).Alternatively, in another embodiment shown in FIG. 30, a bonding forceor pressure (830) may be applied prior to completion of laser doping ofthe base contact to cause an initial approach or contact between thesecond metal foil (782) to the substrate surface. Referring to FIG. 31,laser doping or firing (815) through the second foil (782) may be usedto create contact and doping of the base (364). The dopant atom can comefrom the metal foil (782) or a general dopant material coated on themetal foil (915). From the above noted embodiments, it is understoodthat timing of laser doping is independent from the application bondingforce or pressure and can occur either before, during, or after. Thedopant material (915) may be uniformly coated on the metal foil or metalfoil assembly. Alternatively, it may be coated in selective areas suchas near the opening (914) (e.g. see FIG. 26) to improve performance orminimize dopant usage. In some embodiments, the foil (782) itself may beformed from a chemical composition that supplies a dopant. That dopantmay be the main component of the foil. Alternatively, the dopant may bea minor component of the foil, but with enhanced doping action due toproperties such as volatility or liquid phase diffusion in silicon.

In some embodiments, the second foil (782) may be embossed or patternedin a way to provide elasticity to allow facile deformation (837), suchas depicted in FIG. 27 as a nonlimiting example. Embossing may include,but is not limited to, ridges, protrusions, and other patterns thatallow deformation as they flatten out. The foil may also includeperforations, such as linear perforations or slits, that allowdeformation as they open up. The foil may be patterned with so calledEggbox or Miura motifs to allow expandability. The period of the motifmay be equal to or less than 1000 microns, or equal to or less than 200microns. The formation of deformation (837) may occur once part or allof the metal foil assembly is in place on the substrate. In someembodiments, part or all of the deformation (837) may occur duringfabrication of the multilayer foil assembly prior to bonding to thesubstrate, when the multilayer foil assembly is in roll or sheet form.It may also occur as a separate process step once the multilayer foilassembly is singulated, but prior to bonding to the substrate.

Metal foils or metal foil assemblies may be fabricated, shipped, andsupplied in forms including, but not limited to, wide roll form, slitroll form, sheet form, or singulated. Equipment for the bonding processmay include slitting and chopping equipment to take roll or sheet formand produce metal foil assemblies appropriate for a single solar cell.These operations may take place prior to or after bonding of the foil tothe solar cell.

Non-limiting examples of methods for a metal foil to make connectionwith a doped region (364) on the substrate are discussed herein. For thediscussion herein, FIGS. 26-31 can be referred to as nonlimitingexamples. In some embodiments, the doped region may contain a differentconcentration and/or type of doping atoms than the surroundingsubstrate, or it may be modified in a way to promote a differentconcentration and/or type of charge carriers than the surroundingsubstrate. The doped region may allow effective electrical contact to bemade between the second foil (782) and the substrate at that location.

The doped region (364) may be formed by the addition of a dopant atom tothe silicon substrate. One method includes making a localized dopedregion by thermal diffusion of a dopant atom. In some embodiments, thismay be accomplished by high temperature processing using a doping paste,optionally patterned at or near the doped region (364). In someembodiments, the doped region (364) may be formed by laser doping or ionimplantation.

In yet another embodiment, the doped region may be formed by applyingcharge modifying layers to the silicon substrate. Nonlimiting examplesof such charge modifying layers may be semiconductors or dopedsemiconductors including silicon, amorphous silicon, zinc oxide, andzinc chalcogenides. The charge modifying layers may also be materialswith a fixed charge leading to accumulation layers, including aluminumoxide or silicon nitride. The charge modifying layers may also bemetal-insulator-semiconductor structures (MIS structures), which mayalso exist as tunnel contacts or tunnel oxide contacts.

The doped region (364) may be formed at any suitable point in theprocess, including prior to the bonding of any metal foil or metal foilassemblies to the substrate, or after part or all of a metal foilassembly has been bonded to the substrate. In any of these cases,fiducial marks and alignment methods may be used in order for theresulting doped region (364) to be in sufficient alignment with anopening (914) in the metal foil assembly.

As non-limiting examples, FIGS. 26 and 28 show examples in which thedoped region (364) may exist prior to application of the metal foilassembly. FIGS. 27 and 29 respectively show the process of the secondmetal foil (782) connecting to the pre-existing doped region (364).Alternatively, FIG. 30 shows an example in which a doped region does notexist prior to application of the metal foil assembly. FIG. 31 shows anexample of a process, such as laser doping, to cause the formation ofthe doped region (364) along with connection of the second foil (782) tothe doped region. It is also contemplated that when a doped region (364)exists prior to application of the metal foil, laser firing or laserdoping as shown in FIG. 31 may still be used for purposes includingenhancing the doped region (364) and providing improved conductivecontact of foil (782) to the doped region.

Foil Dielectrics:

Referring to FIGS. 9, 12, and 16 as non-limiting examples, dielectriclayers (778) may comprise any material with low or negligibleconductivity. Such materials may be inorganic materials including SiO₂,Al₂O₃, SiN_(x). The inorganic materials may be applied by liquidcoating, sintering, vacuum coating, anodization, or the like.

In some embodiments, the materials may be polymer films. Polymer filmsmay be applied by solvent coating or casting, or by adhesion ofpreformed polymer films or webs to other functional layers, such asmetal foils.

Nonlimiting examples of polymers suitable for solvent casting includeacrylics including polymethylmethacrylate (PMMA) andpolyethylmethacrylate, polyvinylbutyrals, polyethylenevinylacetate(EVA), poly(vinylchloride) (PVC), poly(cyanoacrylate), and cellulosicsincluding nitrocellulose and cellulose triacetate.

In some embodiments, the dielectric may be equal to or between 2 to 150microns thick, or equal to or between 6 to 40 microns thick.

Nonlimiting examples of polymers suitable for being employed aspreformed films that may serve as dielectric layers include polyestersincluding polyethylene terephthalate (PET) and polyethylene napthalate(PEN), polypropylene, polycarbonate, polyimide, polyvinylidene chloride,polyvinylidene fluoride, and polyvinyl chloride. The films may betreated to improve adhesion, including corona, flame, priming, solutionor vapor chemical, and plasma surface pre-treatments.

The polymer films may be treated or manufactured to provide improvedproperties including strength and dimensional stability. Nonlimitingexamples of treatments include uniaxial orientation, biaxialorientation, and anneals to improve dimensional stability and core set.

Uniaxial and biaxial orientation of the polymer films may be achievedduring manufacture of the film prior to incorporation into a multilayerfoil assembly. Other conventional methods of creating a multilayerstructure, such as sequential deposition on a substrate of metal anddielectric layers, cannot provide the desired film orientation in thepolymer film because significant film orientation cannot be achievedwith alternate deposition methods such as vapor deposition, spraydeposition, or solution casting. These other methods thus cannot providethe desirable properties such as strength associated with an orientedfilm.

In some embodiments, the degree of orientation of the polymer film canbe represented by a planar orientation coefficient, Δp, in accordancewith the equation:

${\Delta\; p} = {\frac{n_{1} + n_{2}}{2} - n_{3}}$in which:

-   -   n₁ is the refractive index in the main direction of orientation        in the plane of the film;    -   n₂ is the refractive index in a direction perpendicular to the        main direction of orientation, in the plane of the film;

and n₃ is the refractive index in a direction perpendicular to the planeof the film.

The refractive indices may be measured by any suitable method, and maybe measured at any wavelength, preferably at the yellow line of sodium(λ=590 nm). A value of Δp=0 would indicate no orientation of the polymerfilm. In some embodiments, the degree of orientation of the polymer filmserving as the dielectric layer may be greater than 0.08, preferablygreater than 0.1.

In some embodiments, foil dielectrics may be transparent. Alternatively,the foil dielectric may include a colorant to allow it to absorb laserradiation and thus increase the efficacy of laser based machining,modification, and ablation processes. Colorants may include dyes andpigments. In some embodiments, the dyes and pigments may have absorptionat 1064 nm, 532 nm or 355 nm. In some embodiments, the colorant may becarbon black in micro- or nano-particle form.

Foil Assembly Rear Contact Solar Cells:

As discussed herein, there numerous nonlimiting variations of foilassembly based rear contact solar cells. FIG. 35 shows a nonlimitingexample of a solar cell structure for a foil assembly based rear contactsolar cell. The solar cell structure includes a front surface (164) forreceiving light and a rear surface (166). In some embodiments, the frontsurface (164) may provide a passivation layer (170) and anti-reflectivelayer (180). In some embodiments, the front surface (164) may betextured to increase anti-reflectivity.

The solar cell structure for a foil assembly based rear contact solarcell provides both contacts on the rear surface (166). While other rearcontact solar cells may utilize non-foil based assemblies, embodimentsdiscussed herein are directed to foil based assemblies. A first metalfoil (772) is in electrical communication with regions of the rearsurface (166). In some embodiments, this rear surface (166) may compriseone or more layers (235) that modify charge carrier concentration, suchas a passivation layer (200) and doped layer (210). In some embodiments,the rear surface (166) may include a rear surface conductive structure(240). The second metal foil (782) is in electrically communication withbase (160) via contacts (917) through layers on the rear surface (166),such as via laser fired contacts. Contact (917) of the second metal foil(782) to the substrate represents a general contact made by any methodor order discussed above.

A second metal foil (782) is placed above and spaced apart from thefirst metal foil (772) by a dielectric layer (778), and the second metalfoil is electrically insulated from the first metal foil to preventshorting or shunting. In some embodiments, many regions may be presentwhere the first metal foil (772) is between the second metal foil (782)and substrate in a direction normal to the surface of the substrate. Inother regions, the first metal foil (772) may include openings (130)through the entirety of the first metal foil layer. In some embodiments,the second metal foil (782) is present at the opening (130) regionsthrough first metal foil (772). In some embodiments, it may be desirablefor the second foil (782) to contain openings (764) through the secondfoil layer below which the first metal foil is present. In someembodiments, the dielectric layer (778) may optionally have openings(135) that partially align with the openings (130) in the first metalfoil. In some embodiments, the openings (135) may have approximatelyequal dimensions to openings (130), but in other embodiments, openings(135) may be smaller than openings (135) to create an overhang (759).The dielectric (778) may or may not be patterned in areas below thesecond foil openings (764). Contact (917) from base (160) to the secondmetal foil (782) is made through an opening (130) to the substrate, suchas by laser firing the contact. Depending on the manufacturing processselected, one or more conductive bonding layers or general boding layers(820, 835 in prior figures) may be present between various layers of thesolar cell structure, and the bonding layers may optionally beconductive.

In some embodiments, the first metal foil (772) is contiguous over aportion of the solar cell with openings (130). In some embodiments, thefraction of area represented by the openings in the first metal foil(772) in a region may be equal or less than 40% of the total areacovered by the foil, equal to or less than 25%, or equal to or less than20%. The plane aspect ratio of an opening (130) can be defined as theratio of the largest width of the opening to the length of the openingin a direction perpendicular to that of the largest width. For example,in reference to FIG. 1, the plane aspect ratio for opening (135) can beviewed as a ratio of the length and width of the shape of said opening,which may be a circle, oval, square, rectangle, or any other suitableshape. The plane aspect ratio of the openings (130) may be equal to orless than 10:1, or equal to or less than 5:1. The vertical aspect ratioof opening (130) may be defined as the ratio of the largest width of theopening in the plane of the foil to the vertical height of the foil orfoil assembly. The vertical aspect ratio of the openings (130) may beequal to or greater than 2:1, or equal to or greater than 5:1.

In some embodiments, the first metal (772) and second metal (782) layersmay have a thickness in the range equal to or between 2 to 100 microns,which may be difficult to achieve with vapor deposition techniques. Insome embodiments, the thickness of the two metal layers may be equal toor between 6 to 70 microns. In some embodiments, the metal layers may becomposed of aluminum, equal to or greater than 60% aluminum, or equal toor greater than 80% aluminum. In some embodiments, the foils may havedifferent textures on each face of a foil. In the manufacturing processof foils, a bright finish may be produced when the foil comes in contactwith a work roller surface. A matte finish may be produced when twosheets are rolled simultaneously, with the sides that are touchingyielding a matte finish. Other mechanical finishing methods can be usedto produce patterns.

Aluminum foils may contain defects typical of their fabricationincluding dislocations such as dislocation slip defects and dislocationglide defects. Vapor deposition of aluminum may have substantially fewerdefects. Solution deposition of aluminum may show a local porosityconsistent with sintering of metal particles.

A foil assembly rear contact solar cell may have tabs (905) in which oneor more of the foils (e.g., 772,782) extend past the edge of thesubstrate (100) and can serve as a point of electrical connection forthe solar cell. The tabs may be integral portions of foils (772, 782)without joints between them and the portion of the foils connected tothe solar cell substrate. The tabs (905) may extend equal to or greaterthan 0.5 mm or equal to or greater than 1.0 mm from the edge of thesubstrate (100). In non-foil contacting approaches, including liquid,vapor, or vacuum deposition techniques, it is difficult to produce tabs(905). Typical contacting with these methods may include a joint of twoor more separate entities.

A non-limiting example focusing on the base contact region in which thesecond foil (782) makes contact to the substrate is discussed herein,which may refer to FIG. 27 as an example. The second foil (782) may havea local deformation (837) of the foil to allow it to approach thesubstrate, which may be achieved utilizing any of the methods discussedabove or a combination thereof. With sufficient bonding force orpressure (830) a contact region (832) can form between the doped region(364), second foil (782), and/or bonding layer (820). Because of surfaceroughness and other considerations, the second foil may not makecontinuous contact in the contact region (832). Contact may beconsidered as regions in which the second foil (782) is within 5 micronsor less from the surface of the substrate. Considering the area of thecontact region (832) in relation to the total area of opening (914), thecontact area may be equal to or between 5% and 80% of the opening area,or between 10% and 50% of the opening area.

In some embodiments, there may be a void area (912) that is defined asan intermediate space created near contact (364) during processing thatis unfilled by any solid or liquid material. In some embodiments, anarea or volume near a metal foil contact (e.g. as shown in FIG. 27) maycontain void areas (912) due to the fact that the original foil hasstiffness and other mechanical properties that prevent it fromconformally and substantially filling opening (914). In someembodiments, the void area may exist between the second metal foil andthe substrate. Further, in some case, the void area may prevent secondfoil (782) from contact with other layers of the MFA (760), such as thedielectric layer, and more importantly, first foil (772). This preventsshunting contact between the second foil (782) and first foil (772). Innon-foil contacting approaches, including liquid, vapor, or vacuumdeposition techniques, it is difficult to such large voids desirable toprevent such shunting issues. The size of a void (912) may becharacterized by its volume or by its projected area in cross section,such as from the view shown in FIG. 27. The cross section may be takenso that it intersects the center of opening (914). In some embodiments,the void projected area for a single base contact of the current foilbased device may be equal to or between 50 μm² to 20,000 μm², or equalto or between 100 μm² to 15,000 μm². In a similar structure with metalapplied by other deposition techniques not using foils, the voidprojected area may be equal to or less than 20 μm², or equal to or lessthan 10 μm². Any suitable technique may be used to measure the totalvolume of void between foils (772) and (782) for a single base contact.In some embodiments, the void volume for a solar cell utilizing thedesign(s) discussed herein may be equal to or between 3×10⁴ μm³ to 2×10⁷μm³. In a similar structure with metal applied by other depositiontechniques not using foils, the void volume associated with a singlecontact may be equal to or less than 5000 μm³, or equal to or less than2000 μm³. The amount of void may also be expressed as a fraction equalto the total volume of void (912) divided by the total volume of theopening (914). In some embodiments, void volume fractions might be inthe range of 3% to 40%. In contrast, a similar structure with metalapplied by other deposition techniques not using foils may have a voidfraction of <0.5%.

Foil assembly rear contact solar cells may have conductive bondinglayers as described above between the first metal foil (772) and rear ofthe substrate (166). The conductive bonding layer may be equal to orgreater than 1 micron in thickness. In non-foil contacting approaches,including liquid, vapor, or vacuum deposition techniques, a discreteconductive bonding layer may not be present and/or a conductive bondinglayer may be thinner than 1 micron.

The following examples are included to demonstrate particular aspects ofthe present disclosure. It should be appreciated by those of ordinaryskill in the art that the methods described in the examples that followmerely represent illustrative embodiments of the disclosure. Those ofordinary skill in the art should, in light of the present disclosure,appreciate that many changes can be made in the specific embodimentsdescribed and still obtain a like or similar result without departingfrom the spirit and scope of the present disclosure.

EXAMPLE

A solar cell employing a multilayer foil assembly was prepared accordingto the following procedure. A 150 μm thick silicon substrate with lightn-type doping was chemically etched with 20% KOH at 80° C. for 12minutes in order to remove saw damage. On the front side, a layer of 10nm of intrinsic a-Si:H (i-a-Si:H) and a layer of 80 nm of SiN_(x) wereapplied by plasma enhanced chemical vapor deposition (PECVD). On therear side, a layer of 10 nm of i-a-Si:H and a layer of 30 nm of borondoped a-Si:H (B-a-Si:H) were applied by PECVD. The combination of thei-a-Si:H and B-a-Si:H forms a silicon heterojunction (SHJ) emitter. Onthe rear, an additional metal deposition comprising 50 nm of aluminumfollowed by 200 nm of silver was deposited by thermal evaporation.

A metal foil assembly comprising a first metal foil of 12 μm aluminumbonded to a 12 μm PET dielectric was laser patterned to produce an arrayof 0.5 mm holes through both the aluminum foil and dielectric on a 1 mmpitch. A bonding agent comprising poly(vinyl butyral) in methyl isobutylketone (MIBK) solvent was applied to the rear of the sample and allowedto dry. The metal foil assembly was placed in contact with the rear ofthe sample and bonding agent, and subjected to a vacuum laminationprocess involving heating the assembly to 180° C. for approximately 10seconds. This process resulted in conductive bonding of the metal foilassembly to the substrate, and more particularly the first metal foil tothe substrate. The sample was then sonicated in MIBK for 60 seconds toremove excess bonding agent, and subjected to a 3 minute PAN etch toremove silver/aluminum exposed in holes of the metal foil assembly,where the metal foil assembly was as an etch mask.

A supplemental insulator comprising the photoresist SU-8/2002 withapproximately 1.8% carbon black (dry film basis) was applied and curedon the sample. The insulator was patterned by ablation using 1064 nmlaser pulses with 100 ns duration. The ablated areas comprisedapproximately 200 μm square regions near the center of the holes in themetal foil assembly. A liquid phosphorus source (Filmtronics P509,diluted 1:14 in ethanol) was applied to the sample and allowed to dry. Asecond metal foil was then applied to the back of the sample, andpressed to contact with the substrate through the holes of the metalfoil assembly using a mechanical press. The sample was then laser firedby applying 1064 nm laser pulses with 600 ns duration to the secondmetal foil in the region of the patterned supplemental insulator,simultaneously producing doped regions and electrical contact betweenthe second metal layer and the substrate.

The resulting sample contains a first metal foil connected to theemitter formed by the i-a-Si:H/B-a-Si:H heterojunction structure, and asecond metal foil, isolated from the first, connected to the n-typebase. The sample is constructed such that the first metal foil extendspast the edge of the sample on one side (Tab-1), while the second metalfoil extends past the edge of the sample on the opposite side (Tab-2) toaid ease of connecting solar cells in series (e.g. FIG. 34). The samplewas tested for IV characteristics between Tab-1 and Tab-2 duringillumination of 1 sun, and yielded an open circuit voltage of 0.609V, ashort circuit current of 37.4 mA/cm², and a fill factor of 0.54.

Implementations described herein are included to demonstrate particularaspects of the present disclosure. It should be appreciated by those ofskill in the art that the implementations described herein merelyrepresent exemplary implementation of the disclosure. Those of ordinaryskill in the art should, in light of the present disclosure, appreciatethat many changes can be made in the specific implementations describedand still obtain a like or similar result without departing from thespirit and scope of the present disclosure. From the foregoingdescription, one of ordinary skill in the art can easily ascertain theessential characteristics of this disclosure, and without departing fromthe spirit and scope thereof, can make various changes and modificationsto adapt the disclosure to various usages and conditions. Theimplementations described hereinabove are meant to be illustrative onlyand should not be taken as limiting of the scope of the disclosure.

What is claimed is:
 1. A method for forming all back contacts for asolar cell, the method comprising: selecting a substrate, wherein a backsurface of the substrate contains a passivation layer; forming amultilayer foil assembly (MFA), wherein the MFA comprises a metal layer,and a dielectric layer in contact with the metal layer, wherein thedielectric layer is patterned with dielectric layer openings, and aformed MFA is freestanding; creating first doping regions on thesubstrate; coupling the formed MFA to the back surface of the substrate;and laser firing after coupling of the MFA and substrate to createsecond doping regions aligned with the dielectric layer openings.
 2. Themethod of claim 1, wherein the metal layer is a first metal layer thatis positioned below the dielectric layer, patterned with first metallayer openings, and placed in contact with the first doping regions, andthe first metal layer openings are aligned with the dielectric layeropenings of the dielectric layer, and the solar cell further comprises asecond metal layer, wherein the dielectric layer is sandwiched inbetween the first metal layer and the second metal layer.
 3. The methodof claim 2, wherein the MFA further comprises the second metal layer. 4.The method of claim 2, wherein the dielectric layer openings have asmaller diameter than the first metal layer openings.
 5. The method ofclaim 2, wherein the second metal layer is patterned with openingsaligned with the first doping regions.
 6. The method of claim 5, whereinthe dielectric layer is patterned with openings aligned with the firstdoping regions.
 7. The method of claim 6, wherein the step of creatingthe first doping regions comprises laser firing the first metal layer ofthe MFA at the first doping regions before the second metal layer isadded to the MFA, wherein first laser fired contacts are formed betweenthe first metal layer and the first doping regions.
 8. The method ofclaim 2, wherein the laser firing occurs through the second metal layer,the dielectric layer openings, and the first metal layer openings,thereby causing the second metal layer to be in electrical contact withthe second doping regions.
 9. The method of claim 2 further comprisingthe step of: applying a supplemental insulating layer before the secondmetal layer is added to the MFA.
 10. The method of claim 2 furthercomprising the step of: etching the substrate to remove a portion or allof a rear conductive structure between the first metal layer and thesubstrate, wherein when the first metal layer is bonded to the substrateprior to the dielectric layer, the first metal layer acts as mask forthe etching step, or when the first metal layer and the dielectric layerare bonded to the substrate simultaneous, the dielectric layer and thefirst metal layer act as a mask for the etching step.
 11. The method ofclaim 2, wherein the second metal layer is coated with a dopant to aidformation of the second doping regions.
 12. The method of claim 11,wherein the dopant is an n-type dopant for silicon.
 13. The method ofclaim 11, wherein the dopant is present on the second metal layer in theregion of the first metal layer openings and the dielectric layeropenings.
 14. The method of claim 2 further comprising the step of:locally deforming the second metal layer in areas corresponding to thedielectric area openings to cause the second metal layer to contact thesecond doped regions, wherein the deforming is performed with laserpulses or by applying pressure or force.
 15. The method of claim 2,wherein the MFA is formed by coupling the first metal layer to thedielectric layer during roll-to-roll processing.
 16. The method of claim1 further comprising the step of: perforating the metal layer to formfirst metal layer openings, wherein the metal layer is a first metallayer and the first metal layer openings are aligned with the dielectriclayer openings of the dielectric layer.
 17. The method of claim 1,wherein the rear surface of the substrate comprises doping layers orjunction inducing layers for a heterojunction or a tunnel junction. 18.The method of claim 1, wherein the MFA further comprises one or morebonding layers to aid adhesion of the MFA to the substrate or to aidadhesion of individual layers of the MFA to each other.
 19. The methodof claim 1, wherein the metal layer of the MFA is a second metal layer;and the method further comprises the step of: coupling a first metallayer to the back surface of the substrate prior to the step of couplingof the MFA to the back surface, wherein the first metal layer ispatterned with first metal layer openings, and the first metal layeropenings are aligned with the dielectric layer openings of thedielectric layer when the formed MFA is coupled to the substrate.